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/module.nix

@@ -201,6 +201,8 @@ in
   imports = [
     # Merge the inventory file
     { inventory = inventoryLoaded; }
+    # Merge the meta attributes from the buildClan function
+    { inventory.meta = if config.meta != null then config.meta else { }; }
   ];
 
   inherit nixosConfigurations;