compositor: Generalize module loading

We can now load any number of general modules, and the shell and xwayland
are just two of them.  We continue to use the mechanism for testing but
custom input drivers or logging mechanisms, for example are other use cases.

tests/weston-test

@@ -1,4 +1,4 @@
 #!/bin/sh
 
-../src/weston --module=$abs_builddir/.libs/${1/.la/.so}
+../src/weston --modules=$abs_builddir/.libs/${1/.la/.so}