Various stubtest fixes (#5215)

stdlib/pkgutil.pyi

@@ -21,7 +21,7 @@ else:
 def extend_path(path: Iterable[str], name: str) -> Iterable[str]: ...
 
 class ImpImporter:
-    def __init__(self, dirname: Optional[str] = ...) -> None: ...
+    def __init__(self, path: Optional[str] = ...) -> None: ...
 
 class ImpLoader:
     def __init__(self, fullname: str, file: IO[str], filename: str, etc: Tuple[str, str, int]) -> None: ...