Git Submodule Support

[Brett-Best]

Adds git submodule support, this allows Mint to install https://github.com/apple/swift-protobuf.

I considered adding this to the tests but it would increase how long they take to run substantially.

I've left Allow edits by maintainers on so you can tweak this if needed @yonaskolb 😄

[aduuub]

This would be great for our use case too!

[yonaskolb]

Thanks @Brett-Best!