Compartmantalize `modules.system` and `modules.usrEnv`

[NotAShelf]

Recently I've moved some of the usrEnv options to meta as a read-only interface. I should refactor modules.systems.{programs, services} to offload userspace programs and services to usrEnv as a way of keeping track of which service goes into home-manager and what goes into NixOS.

This is mostly inspired by the fact that there are two mpd services, one for hm and one for nixos.