Fixes#2379.
The new name was a misnomer because the values really are X11 video
drivers (e.g. ‘cirrus’ or ‘nvidia’), not OpenGL implementations. That
it's also used to set an OpenGL implementation for kmscon is just
confusing overloading.
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.
