This removes the need to have multiple downloads in the stdenv
bootstrap process (like a separate busybox binary for Linux, or
curl/mkdir/sh/bzip2 for Darwin). Now all those files can be combined
into a single NAR.
This makes it consistent with the Nixpkgs fetchurl and makes it work
in chroots. We don't need verification because the hash of the result
is checked anyway.
This ensures that 1) the derivation doesn't change when Nix changes;
2) the derivation closure doesn't contain Nix and its dependencies; 3)
we don't have to rely on ugly chroot hacks.
