lib/modules: Allow an "anonymous" module with key in disabledModules

This makes the following work

    disabledModules = [ foo.nixosModules.bar ];

even if `bar` is not a path, but rather a module such as

    { key = "/path/to/foo#nixosModules.bar"; config = ...; }

By supporting this, the user will often be able to use the same syntax
for both importing and disabling a module. This is becoming more relevant
because flakes promote the use of attributes to reference modules. Not
all of these modules in flake attributes will be identifiable, but with
the help of a framework such as flake-parts, these attributes can be
guaranteed to be identifiable (by outPath + attribute path).

lib/tests/modules/disable-module-with-toString-key.nix

@@ -0,0 +1,34 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+
+  moduleWithKey = {
+    key = 123;
+    config = {
+      enable = true;
+    };
+  };
+in
+{
+  options = {
+    positive = mkOption {
+      type = types.submodule {
+        imports = [
+          ./declare-enable.nix
+          moduleWithKey
+        ];
+      };
+      default = {};
+    };
+    negative = mkOption {
+      type = types.submodule {
+        imports = [
+          ./declare-enable.nix
+          moduleWithKey
+        ];
+        disabledModules = [ 123 ];
+      };
+      default = {};
+    };
+  };
+}