[entity] Reimplement and deprecate entity.cw_metainformation
It shouldn't rely anymore on cnx.entity_metas which is itself deprecated, and
the cw_metainformation API is similarly not needed anymore, access directly to
cwuri / cw_source if needed.
Related to #15538288
from yams.buildobjs import RelationDefinition
class ARelation(RelationDefinition):
subject = 'CWUser'
object = 'CWGroup'