Add fallback to infer top-level names for a distribution. Fixes #330.

importlib_metadata/__init__.py

@@ -1013,6 +1013,18 @@ def packages_distributions() -> Mapping[str, List[str]]:
     """
     pkg_to_dist = collections.defaultdict(list)
     for dist in distributions():
-        for pkg in (dist.read_text('top_level.txt') or '').split():
+        for pkg in _top_level_declared(dist) or _top_level_inferred(dist):
             pkg_to_dist[pkg].append(dist.metadata['Name'])
     return dict(pkg_to_dist)
+
+
+def _top_level_declared(dist):
+    return (dist.read_text('top_level.txt') or '').split()
+
+
+def _top_level_inferred(dist):
+    return {
+        f.parts[0] if len(f.parts) > 1 else f.with_suffix('').name
+        for f in dist.files
+        if f.suffix == ".py"
+    }