Summary:
Added infra for writing and using custom eslint rules and created a rule for disallowing cross-package file imports. Such imports are anti-pattern and they also break standalone plugin bundling.
We still have a bunch of places where Flipper core references code directly from plugins. I've ignored these places for now and added task T71355623 to revisit them.
Reviewed By: passy
Differential Revision: D22998955
fbshipit-source-id: d04cff8fc115ba1300a7e6830306ec134046e927
