Motivation for this change

Alternative attempt to resolve libGL hell, via smooth transition to libglvnd
(this is first step, where no hashes shoudl be changes (or at least minimise rebuild as possible).

(second commit to chromium/default.nix) marked as treewide is just placeholder, I plan to have 3 commits replacing mesa-* with libGL* everywhere.

So next step will provide function to make derivation for /run/opengl-drivers, something, which can be useed as:

mkOpenglDriversSet {
    libGL_provider = mesa_drivers;
    libGLvnd_drivers = [];
    vulcan_drivers = [];
    opencl_drivers = [];
}

or

mkOpenglDriverSet {
    libGL_provider = libglvnd;
    libGLvnd_drivers = [ mesa_drivers_glvnd nvidia_drivers ...];
    vulcan_drivers = [ some_package_providing_icd_file ];
    opencl_drivers = [ some_package_providing_icd_file ];
}

At some moment, we can swtich default libGL alias (introduced in first commit) to libglvnd, and it will be single mass-rebuild. Folks, who have runtime issues with libglvnd, can use first of examples above to use old-style libGL.so.

@vcunat @gnidorah Any thoughts?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.