[package] make GlobGroup a public concept (#56238)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56238
It's already functionally public due to `extern` and `mock`, but
exposing the underlying implementation makes extending PackageExporter
easier.
Changed the underscores, expose on `torch.package`, add docs, etc.
Differential Revision: D27817013
Test Plan: Imported from OSS
Reviewed By: Lilyjjo
Pulled By: suo
fbshipit-source-id: e39199e7cb5242a8bfb815777e4bb82462864027
