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

lib/modules/inventory/constraints/interface.nix

@@ -0,0 +1,66 @@
+{
+  allRoles,
+}:
+{
+  lib,
+  ...
+}:
+let
+  inherit (lib) mkOption types;
+  rolesAttrs = builtins.groupBy lib.id allRoles;
+in
+{
+  options.roles = lib.mapAttrs (
+    _name: _:
+    mkOption {
+      description = ''
+        Sub-attributes of `${_name}` are constraints for the role.
+      '';
+      default = { };
+      type = types.submoduleWith {
+        modules = [
+          {
+            options = {
+              max = mkOption {
+                type = types.nullOr types.int;
+                default = null;
+                description = ''
+                  Maximum number of instances of this role that can be assigned to a module of this type.
+                '';
+              };
+              min = mkOption {
+                type = types.int;
+                default = 0;
+                description = ''
+                  Minimum number of instances of this role that must at least be assigned to a module of this type.
+                '';
+              };
+            };
+          }
+        ];
+      };
+    }
+  ) rolesAttrs;
+
+  # The resulting assertions
+  options.assertions = mkOption {
+    visible = false;
+    default = { };
+    type = types.attrsOf (
+      types.submoduleWith {
+        modules = [
+          {
+            options = {
+              assertion = mkOption {
+                type = types.bool;
+              };
+              message = mkOption {
+                type = types.str;
+              };
+            };
+          }
+        ];
+      }
+    );
+  };
+}