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/default.nix

@@ -11,6 +11,7 @@
   directory, # The directory containing the machines subdirectory # allows to include machine-specific modules i.e. machines.${name} = { ... }
   # A map from arch to pkgs, if specified this nixpkgs will be only imported once for each system.
   # This improves performance, but all nipxkgs.* options will be ignored.
+  # deadnix: skip
   inventory ? { },
   ## Sepcial inputs (not passed to the module system as config)
   specialArgs ? { }, # Extra arguments to pass to nixosSystem i.e. useful to make self available # A set containing clan meta: name :: string, icon :: string, description :: string
@@ -27,14 +28,9 @@ let
       ;
     self = directory;
   };
-  meta = attrs.meta or { };
-  rest = builtins.removeAttrs attrs [
-    "meta"
-    "specialArgs"
-  ];
+  rest = builtins.removeAttrs attrs [ "specialArgs" ];
 in
 eval {
-  inventory.meta = lib.mapAttrs (_: lib.mkDefault) meta;
   imports = [
     rest
     # implementation