Tag snapshot input for association with dep files

Summary:
[Doc](https://docs.google.com/document/d/1-zhGPkmt3I2os-kcIaFk9SvT6odt2AJWswSY-J_6AWs/edit?usp=sharing)

### Context
We are exploring the integration of the Kotlin Incremental Compiler into Buck for the incremental compilation of Kotlin source files. The goal is to improve build speed by only recompiling modified source files within a target, rather than rebuilding the entire target as is currently done.

### Problem
With the incremental compiler enabled, we observed a 14% decrease in LOCAL_DEP_FILE action execution type, compensated by a 10% increase in LOCAL_WORKER execution type ([scuba](https://fburl.com/scuba/buck2_actions_user/amr07tu2)).

### Solution
It was discovered that the new library artifact, classpath snapshots (added in D62533759), used by the compiler were not tagged for consideration by dep_files (see [doc](https://www.internalfb.com/intern/staticdocs/buck2/docs/rule_authors/dep_files/)). As a result, every change in a dependency that resulted in a new jar, even if the change itself was filtered out by dep_files, caused a rebuild of the target since the snapshot of the jar was always evaluated as a change in the input.

### This diff
Tags snapshot inputs for association with dep files.

Reviewed By: IanChilds

Differential Revision: D66533725

fbshipit-source-id: 795e793e21f2bfbf71abe0e195d075a2e3ad1586

prelude/jvm/cd_jar_creator_util.bzl

@@ -290,7 +290,11 @@ def encode_base_jar_command(
         compiling_classpath = classpath_jars_tag.tag_artifacts(
             [dep.abi for dep in compiling_deps_list],
         )
-        compiling_classpath_snapshot = {dep.abi: dep.abi_jar_snapshot or "" for dep in compiling_deps_list} if is_incremental else {}
+
+        # The snapshot inputs are tagged for association with dep_files, but they are not marked as used,
+        # as they serve the incremental compiler's internal needs,
+        # which are utilized after the build system has determined whether a rebuild is necessary.
+        compiling_classpath_snapshot = classpath_jars_tag.tag_artifacts({dep.abi: dep.abi_jar_snapshot or "" for dep in compiling_deps_list}) if is_incremental else {}
 
     build_target_value = struct(
         fullyQualifiedName = qualified_name,