haskell: make `ghc`, `cabal-install`, and `stack` visible

Thanks to @peti. Close #10035.

doc/haskell-users-guide.xml

···
     registered on
     <link xlink:href="http://hackage.haskell.org/">Hackage</link>, but
     strangely enough normal Nix package lookups don't seem to discover
-    any of them:
+    any of them, except for the default version of ghc, cabal-install, and stack:
   </para>
   <programlisting>
-$ nix-env -qa cabal-install
-error: selector ‘cabal-install’ matches no derivations
-
-$ nix-env -i ghc
-error: selector ‘ghc’ matches no derivations
+$ nix-env -i alex
+error: selector ‘alex’ matches no derivations
+$ nix-env -qa ghc
+ghc-7.10.2
 </programlisting>
   <para>
     The Haskell package set is not registered in the top-level namespace

nixos/doc/manual/release-notes/rl-1509.xml

···
 
 <listitem>
   <para>
-    Haskell packages can no longer be found by name, i.e. the commands
-    <literal>nix-env -qa cabal-install</literal> and <literal>nix-env -i
-    ghc</literal> will fail, even though we <emphasis>do</emphasis> ship
-    both <literal>cabal-install</literal> and <literal>ghc</literal>.
+    Haskell packages can no longer be found by name, except for
+    <literal>ghc</literal>, <literal>cabal-install</literal>, and
+    <literal>stack</literal>, even though we do package the whole Hackage.
     The reason for this inconvenience is the sheer size of the Haskell
     package set: name-based lookups such as these would become much
     slower than they are today if we'd add the entire Hackage database

pkgs/top-level/all-packages.nix

···
   haskellPackages = haskell.packages.ghc7102.override {
     overrides = config.haskellPackageOverrides or (self: super: {});
   };
+  inherit (haskellPackages) ghc cabal-install stack;
 
   haxe = callPackage ../development/compilers/haxe {
     inherit (ocamlPackages) camlp4;