fix: Remove incorrect ./parts/fleet-data.nix import from flake.nix and use correct flake-parts structure

parts/nixos-modules.nix

@@ -0,0 +1,5 @@
+# Expose host type modules and installer modules for external use
+{ inputs, ... }:
+{
+  flake.nixosModules = import ../installer/modules.nix { inherit inputs; };
+}