ZODB Components: Products import compatibility
@nexedi This implements Products import compatibility based on
source_reference using existing import hooks so that
Products.ERP5.Document.Person for example is importable.
This only works with non-FS modules (IOW when the ZODB is accessible and portal_components is available).
This is a simplified version of !1130 (not requiring a patch to Zope and thus not working for filesystem modules) and also implements import compatibility for
Products.ERP5Type.Document (!1240 (closed)).