Support for libraries

[MkuuWaUjinga]

Currently, gpt-migrate only works with code repositories that run as applications with a main file as entry point. However, many code bases are libraries. Libraries are different in that they:

• Don't have a dedicated entry point
• Often come with a test suite

The migration problem is equally important, so I think it'd be great if gpt-migrate could support this. For that, we'd have to allow for:

• Several entry points (or no entry point and a separate logic to determine all sources of the dependency DAG)
• The possibility to specify your own test suite (and migrate it to the new target repo if possible)

[joshpxyne]

This is really great! Definitely worth taking a swing at!