Move the code from bin/dg to distgen/bin.py

[bkabrda]

We should move code from bin/dg to a file like distgen/bin.py and use setuptools' entrypoint for the dg binary. This would move all functionality to the distgen library, hence it would be importable, more usable and also testable. This change shouldn't break anything for our users.

[praiskup]

I like the testability POV, +1. I dislike the re-usability POV on bin/dg code because that's (a) something nobody should depend on and thus (b) taking care of the API (==making it a library) would be IMO just wasting of time.