The files are very self-similar, and provided upstream doesn’t do
something completely crazy, they should stay that way.
dhall-json is more complicated, and it wasn’t worth it implemented
something more complicated in build.nix, so for now it stays the same.
GitHub's source tarballs are created in a non-deterministic way and the
order the entries are added is not stable.
In nixpkgs, there is fetchFromGitHub which works around this issue by
being a wrapper around fetchzip and since we're already using the full
URL to the corresponding GitHub archives, I switched to fetchzip instead
to keep the changes minimal.
Signed-off-by: aszlig <aszlig@nix.build>