We just call them a generic name rather than a plugin name. Maybe we
should do two things:
* The @kind in gs_utils_get_cache_filename() should be the plugin name
* The vername in gs_utils_get_cache_filename should just go
This means we'd create /home/hughsie/.cache/gnome-software/fwupd
rather than /home/hughsie/.cache/gnome-software/3.27/firmware
I think trying to manage the cache of the plugin outside of the plugin
is kinda crazy, i think each plugin just needs to be responsible for
cleaning up the cache if required at a good time, perhaps in the
_refresh() callback.