It might not be available right on initialization time if X11 is started
asynchronously. As this is a requirement for our tests, ensure it is there
before proceeding with the test.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/944
This is the filename convention you get when you define a shared module
in meson, and since there is no particular reason to not include the
"lib" prefix, lets make it easier to port it over. While at it,
de-duplicate the retrieval of the plugin name.
