reduce amount of installed packages when activating xserver.enable

[fogti]

I use wayland/sway for basically all desktop work, but the nixos module for my display manager (lightdm) requires that xserver.enable is set to true. This causes environment.systemPackages to be kinda polluted with packages like xterm, which I didn't even use when primarily using XMonad before.

It would be nice if the set of packages which are installed when xserver.enable is set to true could be split into a subset installed into environment.systemPackages and environment.defaultPackages. (so that if the user wants to reduce the installed packages to a more bare-minimum, they can just override environment.defaultPackages = lib.mkForce [];). This would require a change to the definition of defaultPackages, so that, instead of supplying a default value, it would just supply the set with "somewhat below normal" override priority, which would be needed to be replicated across all modules which would utilize this (currently there are afaik none besides environment).

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:
  - environment
  - services.xserver.enable
  - services.xserver.displayManager.lightdm

[Artturin]

#139984

[SuperSamus]

Partially solved by #168194.