flatpak: Fix warning of dupd identifiers in the url
We would get the following URL for VLC: appstream://org.videolan.vlc?alt=org.videolan.vlc,vlc.desktop
This is because the appstream id is org.videolan.VLC but QUrl always makes the host lowercase. This changes to put the id in the path part of the URL so we can treat them separately in AppStreamUtils::appstreamIds, which is the centralised place we have to extract ids in Discover.