[schema deserial] fix loading of schema when mapping an entity type to an existing one To ensure rdef of the old entity type don't overwrite those already existing on the mapped entity type. Also rename some variables/arguments on the way to make things clearer.

# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s specific hooks and operations"""