Comment by aiono

> 1. If the code implementing the module API is private, it must all be colocated in one package. If it is public, then anyone can import it, breaking the module boundary. You need a visibility system that can say "this subtree of packages can cooperate with each other, but code outside the subtree can only use the top-level package."

This is easily achieved in Scala with a particular use of package paths. You are allowed to make some symbols only visible under a package path.