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.

doc/haskell-users-guide.xml

@@ -92,18 +92,18 @@ error: attribute ‘haskellPackages’ in selection path
   </para>
   <programlisting>
 $ nix-env -qaP coreutils
-nixos.pkgs.coreutils  coreutils-8.23
+nixos.coreutils  coreutils-8.23
 </programlisting>
   <para>
     If your system responds like that (most NixOS installatios will),
     then the attribute path to <literal>haskellPackages</literal> is
-    <literal>nixos.pkgs.haskellPackages</literal>. Thus, if you want to
+    <literal>nixos.haskellPackages</literal>. Thus, if you want to
     use <literal>nix-env</literal> without giving an explicit
     <literal>-f</literal> flag, then that's the way to do it:
   </para>
   <programlisting>
-$ nix-env -qaP -A nixos.pkgs.haskellPackages
-$ nix-env -iA nixos.pkgs.haskellPackages.cabal-install
+$ nix-env -qaP -A nixos.haskellPackages
+$ nix-env -iA nixos.haskellPackages.cabal-install
 </programlisting>
   <para>
     Our current default compiler is GHC 7.10.x and the