[package] Introduce Importer to manage module namespace collisions. (#51975)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51975
See comments in code.
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D26340592
Pulled By: suo
fbshipit-source-id: 61b16bafad15e19060710ad2d8487c776d672847
