Support old-style AppStream files that rely on merging

Old-style AppStream 'override' files have the following structure:

  <component type=desktop>
    <id>org.kde.amarok.desktop</id>
    <categories>
      <category>AudioVideo</category>
      <category>Featured</category>
    </categories>
  </component>

We never want to return these as search results as they have a unique ID of
'*/*/*/desktop/org.audacityteam.Audacity/*' which causes chaos if more than one
plugin can refine the desktop ID.

Fixes half of #699
1 job for wip/hughsie/wildcard-legacy in 19 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #355017
build-gnome-software

00:19:10