Add ./pants backends to list all available backend packages (pantsb…

…uild#9468)

Closes pantsbuild#8992.

![example output](https://user-images.githubusercontent.com/14852634/78420351-18b95500-7603-11ea-8250-0a3a2fea7298.png)

[ci skip-rust-tests]  # No Rust changes made.
[ci skip-jvm-tests]  # No JVM changes made.

contrib/avro/src/python/pants/contrib/avro/register.py

@@ -1,6 +1,8 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Code generation from the Apache Avro format into Java targets."""
+
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/register.py

@@ -1,6 +1,8 @@
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Create AWS Lambdas from Python code."""
+
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/register.py

@@ -1,6 +1,8 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Adds the `buildozer` and `meta-rename` goals to automate changes to BUILD files."""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.buildrefactor.buildozer import Buildozer

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/register.py

@@ -1,6 +1,11 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for the Kythe ecosystem.
+
+See https://www.kythe.io.
+"""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.codeanalysis.tasks.bundle_entries import BundleEntries

contrib/confluence/src/python/pants/contrib/confluence/register.py

@@ -1,6 +1,11 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for Atlassian's Confluence service.
+
+See https://www.atlassian.com/software/confluence.
+"""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.confluence.tasks.confluence_publish import ConfluencePublish

contrib/cpp/src/python/pants/contrib/cpp/register.py

@@ -1,6 +1,9 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for C++."""
+
+
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/errorprone/src/python/pants/contrib/errorprone/register.py

@@ -1,6 +1,11 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Linter for Java.
+
+See https://errorprone.info.
+"""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.errorprone.tasks.errorprone import ErrorProne

contrib/findbugs/src/python/pants/contrib/findbugs/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Linter for Java.
+
+See http://findbugs.sourceforge.net.
+"""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.findbugs.tasks.findbugs import FindBugs

contrib/go/src/python/pants/contrib/go/register.py

@@ -1,6 +1,8 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for Go."""
+
 from pants.build_graph.build_file_aliases import BuildFileAliases, TargetMacro
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/googlejavaformat/src/python/pants/contrib/googlejavaformat/register.py

@@ -1,6 +1,11 @@
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Autoformatter for Java to follow Google's Java Style guide.
+
+See https://github.com/google/google-java-format.
+"""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.googlejavaformat.googlejavaformat import (

contrib/jax_ws/src/python/pants/contrib/jax_ws/register.py

@@ -1,6 +1,9 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate web service client stubs from a Web Services Description Language (WSDL) file for
+calling a JAX-WS web service."""
+
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/mypy/src/python/pants/contrib/mypy/register.py

@@ -1,6 +1,11 @@
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Python type checker.
+
+See http://mypy-lang.org.
+"""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.mypy.tasks.mypy_task import MypyTask

contrib/node/src/python/pants/contrib/node/register.py

@@ -1,6 +1,8 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for JavaScript and Node.js."""
+
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/python/src/python/pants/contrib/python/checks/register.py

@@ -1,6 +1,11 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Deprecated custom Python lints.
+
+Instead, use the V2 backend `pants.backend.python.lint.flake8`.
+"""
+
 from pants.base.deprecated import deprecated_module
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/scalajs/src/python/pants/contrib/scalajs/register.py

@@ -1,6 +1,8 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for Scala.js."""
+
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/scrooge/src/python/pants/contrib/scrooge/register.py

@@ -1,6 +1,11 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Thrift code generator.
+
+See https://github.com/twitter/scrooge.
+"""
+
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.scrooge.tasks.scrooge_gen import ScroogeGen

contrib/thrifty/src/python/pants/contrib/thrifty/register.py

@@ -1,6 +1,11 @@
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Thrift code generator for Android.
+
+See https://github.com/microsoft/thrifty.
+"""
+
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

src/python/pants/backend/awslambda/python/register.py

@@ -1,6 +1,8 @@
 # Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Create AWS Lambdas from Python code."""
+
 from pants.backend.awslambda.common import awslambda_common_rules
 from pants.backend.awslambda.python import awslambda_python_rules
 

src/python/pants/backend/codegen/antlr/java/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Java targets from Antlr3 and Antlr4.
+
+See https://www.antlr.org.
+"""
+
 from pants.backend.codegen.antlr.java.antlr_java_gen import AntlrJavaGen
 from pants.backend.codegen.antlr.java.java_antlr_library import JavaAntlrLibrary
 from pants.build_graph.build_file_aliases import BuildFileAliases

src/python/pants/backend/codegen/antlr/python/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Python targets from Antlr3 and Antlr4.
+
+See https://www.antlr.org.
+"""
+
 from pants.backend.codegen.antlr.python.python_antlr_library import PythonAntlrLibrary
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task

src/python/pants/backend/codegen/grpcio/python/register.py

@@ -1,6 +1,11 @@
 # Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Python targets from Protocol Buffers (Protobufs) and gRPC.
+
+See https://grpc.io.
+"""
+
 from pants.backend.codegen.grpcio.python.grpcio_prep import GrpcioPrep
 from pants.backend.codegen.grpcio.python.grpcio_run import GrpcioRun
 from pants.backend.codegen.grpcio.python.python_grpcio_library import PythonGrpcioLibrary

src/python/pants/backend/codegen/jaxb/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Java targets from Java Architecture for XML Bindings (JAXB).
+
+See https://www.oracle.com/technical-resources/articles/javase/jaxb.html.
+"""
+
 from pants.backend.codegen.jaxb.jaxb_gen import JaxbGen
 from pants.backend.codegen.jaxb.jaxb_library import JaxbLibrary
 from pants.build_graph.build_file_aliases import BuildFileAliases

src/python/pants/backend/codegen/protobuf/java/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Java targets from Protocol Buffers (Protobufs).
+
+See https://developers.google.com/protocol-buffers/.
+"""
+
 from pants.backend.codegen.protobuf.java.java_protobuf_library import JavaProtobufLibrary
 from pants.backend.codegen.protobuf.java.protobuf_gen import ProtobufGen
 from pants.build_graph.build_file_aliases import BuildFileAliases

src/python/pants/backend/codegen/ragel/java/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Java targets from Ragel finite state machines.
+
+See http://www.colm.net/open-source/ragel/.
+"""
+
 from pants.backend.codegen.ragel.java.java_ragel_library import JavaRagelLibrary
 from pants.backend.codegen.ragel.java.ragel_gen import RagelGen
 from pants.build_graph.build_file_aliases import BuildFileAliases

src/python/pants/backend/codegen/register.py

@@ -1,6 +1,8 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Enable this backend to turn on every single codegen backend within `pants.backend.codegen`."""
+
 from pants.backend.codegen.antlr.java.antlr_java_gen import AntlrJavaGen
 from pants.backend.codegen.antlr.java.java_antlr_library import JavaAntlrLibrary
 from pants.backend.codegen.antlr.python.antlr_py_gen import AntlrPyGen

src/python/pants/backend/codegen/thrift/java/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Java targets from Thrift.
+
+See https://thrift.apache.org.
+"""
+
 from pants.backend.codegen.thrift.java.apache_thrift_java_gen import ApacheThriftJavaGen
 from pants.backend.codegen.thrift.java.java_thrift_library import JavaThriftLibrary
 from pants.build_graph.build_file_aliases import BuildFileAliases

src/python/pants/backend/codegen/thrift/python/register.py

@@ -1,6 +1,11 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Python targets from Thrift.
+
+See https://thrift.apache.org.
+"""
+
 from pants.backend.codegen.thrift.python.apache_thrift_py_gen import ApacheThriftPyGen
 from pants.backend.codegen.thrift.python.py_thrift_namespace_clash_check import (
     PyThriftNamespaceClashCheck,

src/python/pants/backend/codegen/wire/java/register.py

@@ -1,6 +1,8 @@
 # Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Generate Java targets from the Wire protocol."""
+
 from pants.backend.codegen.wire.java.java_wire_library import JavaWireLibrary
 from pants.backend.codegen.wire.java.wire_gen import WireGen
 from pants.build_graph.build_file_aliases import BuildFileAliases

src/python/pants/backend/docgen/register.py

@@ -1,6 +1,8 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for generating wiki-style documentation from Markdown and RST."""
+
 from pants.backend.docgen.targets.doc import Page, Wiki, WikiArtifact
 from pants.backend.docgen.tasks.generate_pants_reference import GeneratePantsReference
 from pants.backend.docgen.tasks.markdown_to_html import MarkdownToHtml

src/python/pants/backend/graph_info/register.py

@@ -1,6 +1,9 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Various goals for insights on your project's graph, such as finding the path between any two
+targets."""
+
 from pants.backend.graph_info.tasks.cloc import CountLinesOfCode
 from pants.backend.graph_info.tasks.dependees import ReverseDepmap
 from pants.backend.graph_info.tasks.filemap import Filemap

src/python/pants/backend/jvm/register.py

@@ -1,6 +1,8 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for both Java and Scala."""
+
 from pants.backend.jvm.artifact import Artifact
 from pants.backend.jvm.ossrh_publication_metadata import (
     Developer,

src/python/pants/backend/native/register.py

@@ -1,6 +1,8 @@
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Support for using C and C++ with Python distributions via `ctypes`."""
+
 from pants.backend.native.rules.targets import (
     CLibrary,
     ConanNativeLibrary,

src/python/pants/backend/project_info/register.py

@@ -1,6 +1,8 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Various goals for insights on your project, such as finding a target's dependencies."""
+
 from pants.backend.project_info.rules import dependencies, source_file_validator
 from pants.backend.project_info.tasks.dependencies import Dependencies
 from pants.backend.project_info.tasks.depmap import Depmap

src/python/pants/backend/python/lint/bandit/register.py

@@ -1,6 +1,11 @@
 # Copyright 2020 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Security linter for Python.
+
+See https://bandit.readthedocs.io/en/latest/.
+"""
+
 from pants.backend.python.lint.bandit import rules as bandit_rules
 
 

src/python/pants/backend/python/lint/black/register.py

@@ -1,6 +1,11 @@
 # Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Autoformatter for Python.
+
+See https://black.readthedocs.io/en/stable/.
+"""
+
 from pants.backend.python.lint import python_formatter
 from pants.backend.python.lint.black import rules as black_rules
 

src/python/pants/backend/python/lint/docformatter/register.py

@@ -1,6 +1,11 @@
 # Copyright 2020 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Python autoformatter for PEP257 docstring conventions.
+
+See https://github.com/myint/docformatter.
+"""
+
 from pants.backend.python.lint import python_formatter
 from pants.backend.python.lint.docformatter.rules import rules as docformatter_rules
 

src/python/pants/backend/python/lint/flake8/register.py

@@ -1,6 +1,11 @@
 # Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+"""Linter for Python.
+
+Works with all Flake8 plugins. See https://flake8.pycqa.org/en/latest/.
+"""
+
 from pants.backend.python.lint.flake8 import rules as flake8_rules