Now the default way to define NixOS window manager modules is to use
mkEnableOption to describe the module itself.
In this commit, all files on nixos/modules/services/x11/window-managers
are changed.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
