Until now, only bt5 Extension/Test/Document could be migrated from
filesystem.

From migration dialog, allow to select any Products.ERP5.Document.* (only,
for now) to be migrated. By default, automatically select Products Documents
used by the current bt5 Portal Types (by looking at the mro() of its
erp5.portal_type classes).

Also, to easily identified where it was migrated from, source_reference
is set to 'bt.getTitle():ID' for bt5 Extension/Test/Document and
'Products.ERP5.Document.XXX' for filesystem Products.