bpo-34632: Add importlib.metadata (GH-12547)

Add importlib.metadata module as forward port of the standalone importlib_metadata.

Lib/importlib/_bootstrap_external.py

@@ -1363,6 +1363,58 @@ class PathFinder:
             return None
         return spec.loader
 
+    search_template = r'(?:{pattern}(-.*)?\.(dist|egg)-info|EGG-INFO)'
+
+    @classmethod
+    def find_distributions(cls, name=None, path=None):
+        """
+        Find distributions.
+
+        Return an iterable of all Distribution instances capable of
+        loading the metadata for packages matching the ``name``
+        (or all names if not supplied) along the paths in the list
+        of directories ``path`` (defaults to sys.path).
+        """
+        import re
+        from importlib.metadata import PathDistribution
+        if path is None:
+            path = sys.path
+        pattern = '.*' if name is None else re.escape(name)
+        found = cls._search_paths(pattern, path)
+        return map(PathDistribution, found)
+
+    @classmethod
+    def _search_paths(cls, pattern, paths):
+        """Find metadata directories in paths heuristically."""
+        import itertools
+        return itertools.chain.from_iterable(
+            cls._search_path(path, pattern)
+            for path in map(cls._switch_path, paths)
+            )
+
+    @staticmethod
+    def _switch_path(path):
+        from contextlib import suppress
+        import zipfile
+        from pathlib import Path
+        with suppress(Exception):
+            return zipfile.Path(path)
+        return Path(path)
+
+    @classmethod
+    def _predicate(cls, pattern, root, item):
+        import re
+        return re.match(pattern, str(item.name), flags=re.IGNORECASE)
+
+    @classmethod
+    def _search_path(cls, root, pattern):
+        if not root.is_dir():
+            return ()
+        normalized = pattern.replace('-', '_')
+        matcher = cls.search_template.format(pattern=normalized)
+        return (item for item in root.iterdir()
+                if cls._predicate(matcher, root, item))
+
 
 class FileFinder: