Only add <pkgname> entries when bundles have not already been used

The scribus.desktop file has a hardcoded `X-AppInstall-Package=scribus` which
gets processed, merged, and added to the flatpak *AppStream* metadata.

This causes bad things to happen when the appstream helper code helpfully adds
'scribus' as a bundle source, but libflatpak complains it's not a valid ref.
1 job for <span class="ref-name">wip/hughsie/Flatpak_X-AppInstall-Package</span> in 4 minutes and 35 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #230688
build-gnome-software

00:04:35