read elements/attribs written to extension namespace

Related to validation errors with fdo#31296

Change-Id: I1b76e05a9996b1123db164629475a3f7b07dda69
Reviewed-on: https://gerrit.libreoffice.org/7222
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>