fully parse git context

The package-related sender info (package name, commit hash, etc.) should all be populated via `setuptools_scm` and `git describe`. Most of this gets into the version object in setuptools, but it does not all make it into metadata which is easy to reach when creating a `scarab.VersionSemantic` in the package at runtime.

One possible solution is to leverage writing a `version.py` file with a format statement, in conjunction with a custom `git describe` if needed.