Submodules - revive incubator

[vwxyzjn]

This PR adds submodules linking the separate repositories. We still need a way to work with all other repos at the same time in the same virtual poetry environment.

[cswinter]

What I've been doing to develop cross-repo changes is to just have all the repos checked out locally and then poetry run pip install -e ... any dependencies that I want to work on. Not sure if that works for your use case. Either way, we should probably document this workflow somewhere.