Part 1 of #301908.
This renames the two versions of the Apple Public Source License seen in
nixpkgs; `apsl20` was often confused as being for the widely-used Apache
License 2.0.
This probably hasn’t built for a while. Apple is redirecting to GitHub,
which results in different hashes for cctools and ld64. While I’m fixing
the hashes, I also updated the sources to use `fetchFromGitHub`.