Move packages into git submodules.

[richard457]

I often need to tweak the package to fit the use case, but doing so also with flutterfire_ui as the base package, the problem is that I can't do this because flutterfire_ui is not a repo, what if these packages are independent repo that is submodule in your flutterfire repo? I think this will make it easy for contributors like myself to tweak the package to my liking.