build-inventory: move inventory and inventoryClass into explizitly different folders

lib/modules/inventoryClass/roles-interface.nix

@@ -0,0 +1,85 @@
+{
+  settingsOption ? null,
+  nestedSettingsOption ? null,
+}:
+{ lib, clanLib, ... }:
+let
+  inherit (lib)
+    types
+    ;
+in
+{
+  options = {
+    # TODO: deduplicate
+    machines = lib.mkOption {
+      type = types.attrsOf (
+        types.submodule {
+          options.settings =
+            if nestedSettingsOption != null then
+              nestedSettingsOption
+            else
+              lib.mkOption {
+                default = { };
+                type = clanLib.types.uniqueDeferredSerializableModule;
+              };
+        }
+      );
+      default = { };
+    };
+    tags = lib.mkOption {
+      type = types.attrsOf (types.submodule { });
+      default = { };
+    };
+    settings =
+      if settingsOption != null then
+        settingsOption
+      else
+        lib.mkOption {
+          default = { };
+          type = clanLib.types.uniqueDeferredSerializableModule;
+        };
+    extraModules = lib.mkOption {
+      description = ''
+        List of additionally imported `.nix` expressions.
+
+        Supported types:
+
+        - **Strings**: Interpreted relative to the 'directory' passed to buildClan.
+        - **Paths**: should be relative to the current file.
+        - **Any**: Nix expression must be serializable to JSON.
+
+        !!! Note
+            **The import only happens if the machine is part of the service or role.**
+
+        Other types are passed through to the nixos configuration.
+
+        ???+ Example
+            To import the `special.nix` file
+
+            ```
+            . Clan Directory
+            ├── flake.nix
+            ...
+            └── modules
+                ├── special.nix
+                └── ...
+            ```
+
+            ```nix
+            {
+              extraModules = [ "modules/special.nix" ];
+            }
+            ```
+      '';
+      apply = value: if lib.isString value then value else builtins.seq (builtins.toJSON value) value;
+      default = [ ];
+      type = types.listOf (
+        types.oneOf [
+          types.str
+          types.path
+          (types.attrsOf types.anything)
+        ]
+      );
+    };
+  };
+}