modules: add support for module replacement with disabledModules

This is based on a prototype Nicolas B. Pierron worked on during a
discussion we had at FOSDEM.

A new version with a workaround for problems of the reverted original.
Discussion: https://github.com/NixOS/nixpkgs/commit/3f2566689

lib/tests/modules/default.nix

@@ -3,5 +3,6 @@
 {
   inherit (lib.evalModules {
     inherit modules;
+    specialArgs.modulesPath = ./.;
   }) config options;
 }