Fix imported inheritance cleaning up

Copypasted from https://github.com/mono/CppSharp/pull/392 :

This pull request implements test that contains two libraries, NamespacesBase and NamespacesDerived. The NamespacesDerived is dependent of NamespacesBase. It's main purpose is to test the generation of dependent wrappers.

Currently, it generates successfully the wrapper with driver.Options.DependentNameSpaces.Add("NamespacesBase"); However, the plan is to implement some test cases in future that contain more convoluted (and possibly clashing) namespaces, where just adding a using directive isn't feasible.

This pull request also fixes a bug that is imminent with the test: imported/GenerationKind.Linked classes don't get inherited but ignored.

The bug is mentioned in this issue: https://github.com/mono/CppSharp/issues/388 (Doesn't solve the issue itself yet, though.)