Add shell env to buildHex packages.

This adds erlangPackages.${name}.env, which is usable to launch a shell
with the package and it's dependencies like this:

    nix-shell -A erlangPackages.lager.env

This required:

1) refactoring buildHex to become a fixed-point function,
2) moves output of a package into $out/${name},
3) introduces erlEnv (buildEnv, which links in package and it's deps
   into one directory - a super-simple plagiarization of
   haskellPackages.compiler.ghcWithPackages) and
4) shell function producing a un-buildable derivation to be used by
   nix-shell