[package] Pull out _UnpicklerWrapper into PackageUnpickler (#53049)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53049
This makes our API symmetric--now we have an `Importer` aware Pickler
and Unpickler implementation that have similar interfaces.
Test Plan: Imported from OSS
Reviewed By: Lilyjjo
Differential Revision: D26734593
Pulled By: suo
fbshipit-source-id: 3479437cf6b98e0d6a8aa4907c75f0c61d5495d4
