[3.9] bpo-37741: make importlib.metadata docs discoverable through a module directive. (GH-25415)

Automerge-Triggered-By: GH:jaraco.
(cherry picked from commit 23acadc)

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>

Author: jaraco

Doc/library/importlib.metadata.rst

@@ -7,6 +7,8 @@
 .. module:: importlib.metadata
    :synopsis: The implementation of the importlib metadata.
 
+**Source code:** :source:`Lib/importlib/metadata.py`
+
 .. versionadded:: 3.8
 
 .. note::