Support cross-module entity inheritance with roo.

Both base & subclass, each in different maven projects, ( Widget_Roo_Entity.aj & WidgetSub_Roo_Entity.aj ) both contain this declaration and final method:

@PersistenceContext
transient EntityManager <class name>.entityManager;
...
public static final EntityManager <class name>entityManager() {

So subclassing a base class from another maven project cannot work because of the attempt at overriding the final method in the subclass.
Roo does not currently look in dependent projects for base classes and will apparently assume that the absence of the base class in the current project implies that thisEntityManager element must be added to the subclass.

it would be preferable to NOT add it as one could always manually add it if needed, but there's no mechanism for removing it other than scrapping the roo code generation for this entity hierarchy or putting them in the same project.