[package] turn MockZipReader into DirectoryReader and add test coverage (#59107)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59107
Adding documentation, test coverage, and a missing method to the `DirectoryReader` class. `DirectoryReader` was previously named `MockZipReader`, and is used for operating on opened package archives via a `PackageImporter`.
Test Plan: Imported from OSS
Reviewed By: SplitInfinity
Differential Revision: D28760410
Pulled By: Lilyjjo
fbshipit-source-id: aa9d0a68e19738a6d5555bb04ce33af6a53f1268
