[package] implicitly extern stdlib before mocking (#49306)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49306
This allows you to mock out everything except for specific patterns while
still correctly externing the python standard library. This makes it less
likely that you will need to override require_module.
Test Plan: Imported from OSS
Reviewed By: suo
Differential Revision: D25526212
Pulled By: zdevito
fbshipit-source-id: 7339f4c7f12af883496f79de95e57d452bb32dc2
