Fix AttributeError on postcommit when db-create adds non-system CWSource entities In this case, `Repository.add_source` is called from an operation, not from `Repository.init_sources_from_database`, thus the attribute `sources_by_eid` does not exist. Closes #16251078.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)