allow extensions to override Access2Base

In particular, this allows users or administrators to upgrade Access2Base by installing the extension version of it.

Change-Id: I1650e5beced0fd168afa029dbac2025e2fa16e1f
Reviewed-on: https://gerrit.libreoffice.org/9754
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>