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>
(cherry picked from commit 9b5b0aca5f33012344fef73449c17f4dfd8c8043)
Reviewed-on: https://gerrit.libreoffice.org/9761
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>