Allow to find qualified packages via RoleBehavior

Sometimes finding qualified packages is to complex to be described in the
roles.xml.

Hence allow the RoleBehavior to override the logic if needed.

Bug: 110557011
Test: Created a role with custom behavior. Looked at old and new roles.
Change-Id: I5c8f6875504cb7e84bc45762227d0649fee0d6b7