Group Flatpaks by installation in gs_plugin_download()

Unfortunately the fix in 3e1fe203 is incomplete: the call to _build_transaction() later in the function also assumes that flatpak is non-NULL.

Apply the same fix as 12c3f646 to gs_plugin_download() – as opposed to gs_plugin_update() – to group the apps by their flatpak installation, and download each group independently.

I refactored the group-by-installation code a little which I think simplifies the two call sites.