Can't get xproto pkg-config file for buildfhsenv

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Hello, does anybody know which package provides “xproto” pkg-config files? I’m trying to build a flutter application which requires it, but it complains about not finding “xproto.pc”. I’ve already tried the xorgproto package from stable and unstable, but without success. I’m using buildfhsenv because it was the easiest solution to get flutter working (couldn’t get it working with other methods after days of experimenting, I’m new to nix and nixos so it’s probably my fault). pkg-config --debug --exists xproto yields nothing.

This is the shell.nix file:

{ pkgs ? import <nixpkgs> {}, unstable ? import <nixos-unstable> {} }:

(pkgs.buildFHSEnv {
  name = "flutter";
  targetPkgs = pkgs: (with pkgs; [
    curl
    git
    unzip
    xz
    zip
    jdk17
    freetype
    fontconfig
    cmake
    ninja
    gtk3
    gtk3.dev
    pango.dev
    gobject-introspection.dev
    glib.dev
    harfbuzz.dev
    cairo.dev
    gdk-pixbuf.dev
    atk.dev
    pcre2.dev
    pkg-config
    libffi.dev
    zlib.dev
    libepoxy.dev
    util-linux.dev
    freetype.dev
    pulseaudio.dev
    libGL
    libGLU
    nss
    nspr
    expat
    alsa-lib
    libselinux.dev
    bzip2.dev
    libpng.dev
    brotli.dev
    graphite2
    libsepol.dev
    pcre.dev
    fribidi.dev
    libthai.dev
    libdatrie.dev
    fontconfig.dev
    expat.dev
    unstable.xorg.xorgproto
  ])++ (with pkgs.xorg; [
    libX11.dev
    libxcb.dev
    xcbproto
    xorgserver.dev
    libXcursor
    libXrandr
    libXrender.dev
    libXext.dev
    libXtst
    libXi
    libXcomposite
    libXdamage
    libXfixes
    libXt.dev
  ]); 
  runScript = "bash";
  profile = "export PATH=\"$PATH:/home/user/android/flutter/bin:/home/user/android/android-studio/bin\"";
}).env