Xscreensaver plugin is never used
After fixing ticket:4271 xscreensaver plugin is still useless, as xdg-screensaver has higher priority and is always used regardless of the actual screensaver in use.
One way to fix this would be to check if the programs used (xdg-screensaver and xscreensaver-command) actually exist and fail the plugin if missing. But that can also pick the wrong one as xdg-screensaver command is distributed separately from the screensavers supporting it.