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`).
