JCasGen shouldn't generate types for imported type systems (or at least it should be an option)

Our project reuses a common type system that we got from a different source. The common type system descriptor is imported into our main type system descriptor. The common type system has its own JCas types, in a jar file.

When we generate JCas types for our main type system descriptor, it currently generates all of the classes for all of the imported type systems as well. We don't want this behavior, so we have to manually go through and delete those classes.

I think JCasGen should only generate types for the type system descriptor that you run it on, not on imported type system descriptors.