fix: set appmap_dir, language in default config

Make sure `appmap_dir` and `language` are set the default configuration,
and therefore in the generated appmap.yml. The IDE extensions will
perform better as a result.

appmap/_implementation/configuration.py

@@ -173,7 +173,23 @@ def labels(self):
 
     @property
     def default(self):
-        return {"name": self.default_name, "packages": self.default_packages}
+        ret = {
+            "name": self.default_name,
+            "language": "python",
+            "packages": self.default_packages,
+        }
+        env = Env.current
+        output_dir = env.output_dir
+        root_dir = env.root_dir
+        try:
+            ret["appmap_dir"] = str(output_dir.relative_to(root_dir))
+        except ValueError:
+            # The only way we can get here is if APPMAP_OUTPUT_DIR is set, and we've already logged
+            # a warning. (Note that PurePath.is_relative_to wasn't added till 3.9, so it can't be
+            # used here.)
+            pass
+
+        return ret
 
     @property
     def default_name(self):

appmap/_implementation/env.py

@@ -43,8 +43,15 @@ def __init__(self, env=None, cwd=None):
         self._root_dir = str(self._cwd) + "/"
         self._root_dir_len = len(self._root_dir)
 
-        output_dir = Path(self.get("APPMAP_OUTPUT_DIR", "tmp/appmap"))
-        self._output_dir = output_dir.resolve()
+        logger = logging.getLogger(__name__)
+        # The user shouldn't set APPMAP_OUTPUT_DIR, but some tests depend on being able to use it.
+        appmap_output_dir = self.get("APPMAP_OUTPUT_DIR", None)
+        if appmap_output_dir is not None:
+            logger.warning("Setting APPMAP_OUTPUT_DIR is not supported")
+        else:
+            appmap_output_dir = "tmp/appmap"
+
+        self._output_dir = Path(appmap_output_dir).resolve()
 
     def set(self, name, value):
         self._env[name] = value

appmap/test/test_configuration.py

@@ -137,6 +137,7 @@ def check_default_config(self, expected_name):
             {"path": "package"},
             {"path": "test"},
         ]
+        assert default_config.default["appmap_dir"] == "tmp/appmap"
 
 
 class TestDefaultConfig(DefaultHelpers):