ZODB Components not following ID=(document|extension|test).VERSION.REFERENCE convention are not importable anymore.

Import mechanism now relies on this convention to find the document in
portal_components.

This is necessary to get rid of _registry_dict (ZODB Components equivalent of
document_class_registry), along with several locks. This used to be a mapping
between (reference, version) to ID stored outside of Zope Transactions as a
dict on global erp5.component.XXX modules.

This did not work for long transactions such as installing many bt5s:
  1. Transaction1: Install a bt5 with ZODB Component "Foo".
     => Trigger a reset and _registry_dict is cleared.
  2. Transaction2: TimerServer kicks in.
     => Generate _registry_dict without "Foo".
  3. Transaction1: Install another bt5 using "Foo".
     => "Foo" not in _registry_dict and thus considered not present.