Move contents of builtins/* to stdlib/*. This simplifies finding stubs.

stdlib/3/atexit.pyi

@@ -0,0 +1,9 @@
+"""Stub file for the 'atexit' module."""
+
+from typing import Any, Callable
+
+def _clear() -> None: ...
+def _ncallbacks() -> int: ...
+def _run_exitfuncs() -> None: ...
+def register(func: Callable[..., Any], *args, **kwargs) -> Callable[..., Any]: ...
+def unregister(func: Callable[..., Any]) -> None: ...