libnm doesn't only search for plugins in the regular VPN plugin directory,
but also in the legacy location and the directory pointed to by the
NM_VPN_PLUGIN_DIR environment variable (if set).
We don't monitor the additional directories, so it's possible for our cache
to become outdated.
Instead of trying to play catch-up with libnm's internals, do what nm-applet
does and use the appropriate API to look up the plugin on each request.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/2386
eb7533bbf1