modulefinder: treat __init__.pyi like __init__.py

The main idea being that we shouldn't consider a directory a namespace
package if either of them is present.

Author: Arne Welzel

mypy/modulefinder.py

@@ -227,6 +227,7 @@ def _find_module(self, id: str) -> Optional[str]:
                         continue
                     return path
                 elif fscache.isfile_case(path_stubs, dir_prefix):
+                    has_init = True
                     if verify and not verify_module(fscache, id, path_stubs, dir_prefix):
                         near_misses.append((path_stubs, dir_prefix))
                         continue