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.

lib/tests/modules/default.nix

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

lib/tests/modules/disable-declare-enable.nix

@@ -0,0 +1,5 @@
+{ lib, ... }:
+
+{
+  disabledModules = [ ./declare-enable.nix ];
+}

lib/tests/modules/disable-define-enable.nix

@@ -0,0 +1,5 @@
+{ lib, ... }:
+
+{
+  disabledModules = [ ./define-enable.nix ];
+}

lib/tests/modules/disable-enable-modules.nix

@@ -0,0 +1,5 @@
+{ lib, ... }:
+
+{
+  disabledModules = [ "define-enable.nix" "declare-enable.nix" ];
+}