Files should not import things public

[mortberg]

Some files import things public which can cause problems later when renaming and refactoring. We should get rid of all public imports that are not crucial (for example the Prelude should expose all of Core.Primitives so that we don't have to import it everywhere). It would be good with a PR which does only this and adds a bullet about it to the style guide

[felixwellen]

We have the bullet point in CONTRIBUTING.md. So it remains to be checked, if there are still public imports that should not be public.