Unify NixOS and Nixpkgs channel structure

This is primarily to ensure that

  -I nixpkgs=https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz

and

  -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz

and

  -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz

behave consistently.

It also allows installing packages via "nix-env -iA nixos.<pkg>"
rather than "nixos.pkgs.<pkg>". It would be even better to allow
"nixpkgs.<pkg>", but that requires a change to nix-channel.

Fixes #7659.

nixos/modules/misc/nixpkgs.nix

@@ -72,7 +72,7 @@ in
   };
 
   config = {
-    _module.args.pkgs = import ../../lib/nixpkgs.nix {
+    _module.args.pkgs = import ../../.. {
       system = config.nixpkgs.system;
 
       inherit (config.nixpkgs) config;