Use Build.Print to show errors
cosmetic: get rid of a (>>) in favor of a do-block
Rename the dummy paths file so that it does not register as C code!
Add check that all ports appear in the "main" module

It is unclear what it means if ports are used in sub-modules, so to be
cautious, we want to rule it out in the first release. As people use
the API it’ll become clear how sub-module ports are necessary and what
problems they might cause.