Fix: forward meta attributes in flake-parts module

This fixes building machines with the flake-parts module.

Since the inventory merges the machines, the `clan.core.meta` attribute
is now a submodule and conditionally imported.

Also drops the following attributes:
- clan.core.meta.icon
- clan.core.meta.description

In favor of specifying them inside the inventory.

This doesn't use `mkRemovedOptionsModule`, because in our case it would
cause infinite recursion.

lib/build-clan/interface.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ lib, self, ... }:
 let
   types = lib.types;
 in
@@ -7,6 +7,7 @@ in
     # Required options
     directory = lib.mkOption {
       type = types.path;
+      default = self;
       description = "The directory containing the clan subdirectory";
     };
 
@@ -26,22 +27,18 @@ in
     };
 
     # Meta
-    meta = {
-      name = lib.mkOption {
-        type = types.nullOr types.str;
-        default = null;
-        description = "Needs to be (globally) unique, as this determines the folder name where the flake gets downloaded to.";
-      };
-      icon = lib.mkOption {
-        type = types.nullOr types.path;
-        default = null;
-        description = "A path to an icon to be used for the clan in the GUI";
-      };
-      description = lib.mkOption {
-        type = types.nullOr types.str;
-        default = null;
-        description = "A short description of the clan";
-      };
+    meta = lib.mkOption {
+      type = types.nullOr (
+        types.submodule {
+          options = {
+            name = lib.mkOption {
+              type = types.nullOr types.str;
+              description = "Needs to be (globally) unique, as this determines the folder name where the flake gets downloaded to.";
+            };
+          };
+        }
+      );
+      default = null;
     };
 
     pkgsForSystem = lib.mkOption {