[migration] Skip virtual rtypes when adding relation definition through add_cube Virtual rtypes should be skipped as they are added dynamically on loading schema. Those are skipped by e.g. schema serialisation or add_entity_type, do the same thing here.

psycopg2
ldap3 < 2
cubicweb-basket
cubicweb-card
cubicweb-comment
cubicweb-file
cubicweb-localperms
cubicweb-tag