Add tests for OsScope and PackagesScope

This tests the API we use throughout the code. So it expresses our
expectations of the API and also serves as a safety net for future
refactorings.
Use Scope as mixin instead of class

Use the ScopeMixin instead of the Scope inheritance. This gets rid
of the delegation.
Merge branch 'move-scope-to-mixin-3'

Pull request #175