Integrate wikidata (composer) built with mediawiki/vendor . Otherwise stuff will break as soon as core and wikidata share a dependency. Because the composer run for mediawiki/core does not know about the composer from the wikidata built. So we need to have one build with one composer run that knows about both parts so it does not duplicate the same namespace.
Oops we yesterday in all the discussions forgot that the autoloader does not try to load a class again that is already loaded. And the next autoloader is only tried until one succeeds. (So as long as nobody uses require/include on a duplicate that will not break.)
We still probably want to have a unified build, because composer then will detect incompatible version requirements.