This new document is a mixture of context that was previously just
institutional knowledge held by me and a few other Terraform Core members,
and of new information I've learned while practicing dependency upgrades
under our new multi-module repository layout.
I've documented this as a starting point for those who are completely new
to the codebase, and for those who were already familiar but haven't yet
performed upgrades in the new multi-module layout.
This all still remains slightly in flux because I'm using real dependency
upgrades to practice different interactions between the different modules,
and so this is likely to evolve as we learn more, but we need to have at
least some initial docs on this around in case we need to perform upgrades
in an emergency situation where I am not around to assist directly.
