use SvXMLExport::GetRelativeReference(), fdo#79305

... instead of a manual approach that missed one extra package level.

Change-Id: I41c76eb84677fcf49eb09830127a419ed23ec643
(cherry picked from commit bc3b62e25eb0c3921fa600e80eeb314e45ecaaef)
Reviewed-on: https://gerrit.libreoffice.org/10444
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>