haskell: add support for all of Hackage

The function "callHackage <name> <version>" generates build instructions for
the requested library version on-the-fly. All of Hackage is available. Note:
this code is brand-new, experimental, and it might change in the future. Don't
base production code on this feature yet.

Example usage:

  $ nix-shell -p 'haskellPackages.callHackage "cpphs" "1.19.3" {}' --run "cpphs --version"
  cpphs 1.19.3

  $ nix-shell -p 'haskellPackages.ghcWithPackages (self: [(self.callHackage "hsdns" "1.6.1" {})])' --run "ghc-pkg list hsdns"
  /nix/store/p6r81k2vb2pzy4wcvri6z9m492i0hg63-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d
      hsdns-1.6.1

pkgs/development/haskell-modules/default.nix

@@ -8,6 +8,8 @@ let
 
   inherit (stdenv.lib) fix' extends;
 
+  inherit (import ./lib.nix { inherit pkgs; }) hackage2nix;
+
   haskellPackages = self:
     let
 
@@ -57,6 +59,8 @@ let
 
         inherit mkDerivation callPackage;
 
+        callHackage = name: version: self.callPackage (hackage2nix name version);
+
         ghcWithPackages = selectFrom: withPackages (selectFrom self);
 
         ghcWithHoogle = selectFrom: