[repository] deprecate the extid2eid based multi-sources API
This API is cumbersome and lead to obfuscated code because of the callback
mecanism implied when some entity has to be created.
Since we dropped the "true" multi-source, this mecanism is not needed anymore
and one should prefer to use the dataimport API inside its parser instead.
Notice the cwxml parser will trigger the deprecation warning, it should not be
used anymore in favor of ad-hoc parsers.