[package] Add `allow_empty` flag to mock and extern (#53232)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53232
**Summary**
This commit adds an optional `allow_empty` argument to
`PackageExporter.mock` and `PackageExporter.extern` that allows certain
patterns for mocked modules and extern modules to be marked ones that
*must* be matched during the packaging process. If a mock or extern
module with `allow_empty=False` is not matched while packaging, an error
is thrown.
**Test Plan**
This commit adds two new test cases to `PackagingTest`,
`test_extern_glob_allow_empty` and `test_mock_glob_allow_empty` that
test this new flag. Existing tests already tests `allow_empty=True`.
**Fixes**
This commit fixes #53217.
Test Plan: Imported from OSS
Reviewed By: suo
Differential Revision: D26834011
Pulled By: SplitInfinity
fbshipit-source-id: 9cf4ea56079ae210d6cfa8604218849eb5cde5f4
