[Nix-dev] Moving some of the NixOS setup to nixpkgs

[宋文武]

hi, I'd like introduce my idea, and wait for feedback.
see my latest comment in PR #4493 for code.

what I do is:
  callPackageModule, a new function to define package.
  using evalModules combined with a package module and config.nix
  to get derivation, the customized package.

  make config.nix aka nixpksg.config just a really module.

what I get is:
  a fully documented config.nix, just like nixos options.
  a module powered package customization method.

yeah, it's just the nixos's modules/configuration.nix
at user's hand but start from scratch.

todo: write or port modules for xdg, gtk, DE and user services.