fex: use packaging for aarch64_fit_native.py

The build's Python env only provided setuptools for the pkg_resources fallback
in Scripts/aarch64_fit_native.py, but modern setuptools / Python 3.14 no longer
ship pkg_resources and packaging was never added, so configure aborted at
string(STRIP ...) (CMakeLists.txt:494). Use packaging, which the script prefers;
libclang (thunk generator) is unchanged.

Assisted-by: Claude Code (Claude Opus 4.8)

pkgs/by-name/fe/fex/package.nix

@@ -185,7 +185,7 @@ llvmPackages.stdenv.mkDerivation (finalAttrs: {
     llvmPackages.bintools
     (python3.withPackages (
       pythonPackages: with pythonPackages; [
-        setuptools
+        packaging
         libclang
       ]
     ))