Extract _topmost and _get_toplevel_name functions.

importlib_metadata/__init__.py

@@ -973,11 +973,34 @@ def _top_level_declared(dist):
     return (dist.read_text('top_level.txt') or '').split()
 
 
+def _topmost(name: PackagePath) -> Optional[str]:
+    """
+    Return the top-most parent as long as there is a parent.
+    """
+    top, *rest = name.parts
+    return top if rest else None
+
+
+def _get_toplevel_name(name: PackagePath) -> str:
+    """
+    Infer a possibly importable module name from a name presumed on
+    sys.path.
+
+    >>> _get_toplevel_name(PackagePath('foo.py'))
+    'foo'
+    >>> _get_toplevel_name(PackagePath('foo.pyc'))
+    'foo'
+    >>> _get_toplevel_name(PackagePath('foo/__init__.py'))
+    'foo'
+    """
+    return _topmost(name) or (
+        # python/typeshed#10328
+        inspect.getmodulename(name)  # type: ignore
+    )
+
+
 def _top_level_inferred(dist):
-    opt_names = {
-        f.parts[0] if len(f.parts) > 1 else inspect.getmodulename(f)
-        for f in always_iterable(dist.files)
-    }
+    opt_names = set(map(_get_toplevel_name, always_iterable(dist.files)))
 
     @pass_none
     def importable_name(name):