cabal2nix: make the package visible from the top-level

This means that "nix-env -i cabal2nix" suffices to install this package.

pkgs/development/haskell-modules/configuration-common.nix

···
   zlib = dontCheck super.zlib;
 
   # Override the obsolete version from Hackage with our more up-to-date copy.
-  cabal2nix = pkgs.cabal2nix;
+  cabal2nix = self.callPackage ../tools/haskell/cabal2nix {};
 
   # https://github.com/urs-of-the-backwoods/HGamer3D/issues/7
   HGamer3D-Bullet-Binding = dontDistribute super.HGamer3D-Bullet-Binding;

pkgs/development/tools/haskell/cabal2nix/default.nix

···
     sha256 = "1w5ba7cdanpq4nr8xngk1jsj0p6b17c6ap24ldzggrln216f3f7d";
     deepClone = true;
   };
-  isLibrary = true;
+  isLibrary = false;
   isExecutable = true;
   libraryHaskellDepends = [
     aeson ansi-wl-pprint base bytestring Cabal containers

pkgs/top-level/all-packages.nix

···
 
   byobu = callPackage ../tools/misc/byobu { };
 
-  cabal2nix = haskellPackages.callPackage ../development/tools/haskell/cabal2nix {};
+  cabal2nix = haskellPackages.cabal2nix;
 
   capstone = callPackage ../development/libraries/capstone { };