Some general purpose tools in haskell-packages.nix have no GHC version string
baked in their name, i.e. alex, haddock, and happy. "nix-env -u" prefers the
first match it finds. Thus, if haskellPackages_ghc6104 et al would have the
same priority as haskellPackages, then nix-env would choose those the old
variants of these tools, because they are listed first.
svn path=/nixpkgs/trunk/; revision=28433