buildClan function: export all machines via nixosModules/darwinModules

We want each machine not only to be exposed via nixosConfigurations but also as a module.
This allows re-importing the machine in tests and override the architecture for example.

lib/build-clan/interface.nix

@@ -162,6 +162,32 @@ in
       default = { };
     };
 
+    nixosModules = lib.mkOption {
+      # Hide from documentation.
+      # Exposed at the top-level of the flake, clan.nixosModules should not used by the user.
+      # Instead, the user should use the `.#nixosModules` attribute of the flake output.
+      visible = false;
+      type = types.lazyAttrsOf types.raw;
+      default = { };
+      description = ''
+        NixOS modules that are generated by clan.
+        These are used to generate the `nixosConfigurations`.
+      '';
+    };
+
+    darwinModules = lib.mkOption {
+      # Hide from documentation.
+      # Exposed at the top-level of the flake, clan.darwinModules should not used by the user.
+      # Instead, the user should use the `.#darwinModules` attribute of the flake output.
+      visible = false;
+      type = types.lazyAttrsOf types.raw;
+      default = { };
+      description = ''
+        Darwin modules that are generated by clan.
+        These are used to generate the `darwinConfigurations`.
+      '';
+    };
+
     # flake.clanInternals
     clanInternals = lib.mkOption {
       # Hide from documentation. Exposes internals to the cli.