[dataimport] make MetaDataGenerator / NoHookObjectStore usable from a datafeed parser
ie make it possible to create entities with external source as cw_source and
proper external id.
Closes #4891552
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py