lib.modules: support strings with absolute paths in disabledModules

This is particularly useful for disabling modules defined in a flake.
Example:
disabledModules = [ "${flake}/modules/mymodule.nix" ];

Previously, absolute string paths were internally prepended with `modulesPath`,
which caused the module filtering to fail.

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

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