[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
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html