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/disable-declare-enable.nix

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