treewide: refactor to use libglvnd

* Implement libGL as a symlink package which uses libraries from libglvnd and
  headers from Mesa (since ones from libglvnd are outdated).
* Use libGL_driver.driverLink treewide; add FHS paths where possible.

pkgs/os-specific/linux/amdgpu-pro/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, elfutils, libGL
+{ stdenv, fetchurl, elfutils
 , xorg, patchelf, openssl, libdrm, libudev
 , libxcb, libxshmfence, epoxy, perl, zlib
 , fetchFromGitHub, ncurses
@@ -13,8 +13,6 @@ let
 
   kernelDir = if libsOnly then null else kernel.dev;
 
-  inherit (libGL) driverLink;
-
   bitness = if stdenv.is64bit then "64" else "32";
 
   libArch =