[package] rename PackageExporter.external to PacakgeExporter.extern_modules (#54601)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54601
This make it consistent with PackageImporter and the on-disk format.
Test Plan: Imported from OSS
Reviewed By: Lilyjjo
Differential Revision: D27296915
Pulled By: suo
fbshipit-source-id: a9bc615b1952b6cc4dcba31d4a33932b1fa1a2aa
