Externalize Archetype Catalog model into separate module

Externalize Archetype Catalog model into separate module (ideally, without any dependency).

This model would be likely reused by 3rd parties, but without "dragging" all the dependencies and other classes in archetype-common module.

Typical example is Nexus's ArchetypePlugin, that publishes the archetype catalog, but from Nexus Index. Currently, that plugin copied (rev668260) of Modello model to be able to publish. This obviously may lead to problems later.