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 continues where d8f7f6a5ce left off. Similarly
to that commit, this commit this also points `sourceRoot`s to `src.name` and similar
instead of keeping hardcoded names, and edits other derivation attrs do do the same,
where appropriate.
Also, similarly to d8f7f6a5ce some of expressions this
edits use `srcs` attribute with customly-named sources, so they have to be moved
into `let` blocks to keep evaluation efficient (the other, worse, way to do this
would to recurcively refer to `elemAt n finalAttrs.srcs` or, similarly, with `rec`).
This reverts commit 6c182dd14d.
No reason to drop working code, for which there is no other alternative
(in this case, formatting/checking HFS+ filesystems).
