Related fdo#82953: Forget package URL of image after it is loaded

It causes problems if we handle those imported images differently which
are identified by a package URL, so after the first load remove
this URL and handle images on the same way as inserted images.

Some related bugs:
* #i44367#
* #i124946#
* #i114361#
* fdo#73270

The image in the test document has a special ID which is different
from that one which is generated by LO internally so after ODF export
the new generated image URL is different from the imported one.

(cherry picked from commit 286e2f5c6ec829bc0987b1be7016699f7ef03e5e)

Change-Id: Ic80adf76c72123af8e89ca3daa4dfa57d7c362a6
Reviewed-on: https://gerrit.libreoffice.org/12503
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>