ArtifactHandler, LifecycleMapping from plugin dependency is not used when plugin extensions are enabled

MNG-3506 fixed the case where two plugins with extensions enabled get in one another's way, and some artifact handlers are not loaded properly.

A related issue is that artifact handlers (and lifecycle mappings) brought in via plugin dependencies are not used. This is because only the plugin's own artifact is actually scanned for extension components like ArtifactHandler or LifecycleMapping.