[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/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl