[package] move importer logic into import pickler (#48632)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/48632
Test Plan: Imported from OSS
Reviewed By: suo
Differential Revision: D25236017
Pulled By: zdevito
fbshipit-source-id: 57fd80d36ddf390ae35c58adf6dddbf15a1347c1
