chore: Regarding the restrictions on package imports, consider introducing linting or documentation.

[Ubugeeei]

I've noticed in several PRs that there are reviews mentioning not to import runtime-core or compiler-core into vapor.

Indeed, it seems this isn't written anywhere, so we should compile the constraints and either lint the imports or include them in the contribution guide.

Even if we introduce linting, I think it might not be clear how to address it, so documentation is probably better. (Something along the lines of 'please implement by copying').

[Ubugeeei]

@LittleSound
Thanks! c82ed7d