fixup! Search sys.path for PEP-561 compliant packages

Author: AWhetter

mypy/modulefinder.py

@@ -242,7 +242,9 @@ def _find_module_non_stub_helper(self, components: List[str],
         dir_path = pkg_dir
         for index, component in enumerate(components):
             dir_path = os.path.join(dir_path, component)
-            if self.fscache.isfile(os.path.join(dir_path, 'py.typed')):
+            if not self.fscache.isdir(dir_path):
+                break
+            elif self.fscache.isfile(os.path.join(dir_path, 'py.typed')):
                 return os.path.join(pkg_dir, *components[:-1]), index == 0
             elif not plausible_match and (self.fscache.isdir(dir_path)
                                           or self.fscache.isfile(dir_path + ".py")):