This introduces the `pkgs/by-name` directory as proposed by RFC 140.
Included are:
- The implementation to add packages defined in that directory to the
top-level package scope
- Contributer documentation on how to add packages to it
- A GitHub Actions workflow to check the structure of it on all PRs
- Better filesystem case-sensitivity heuristic
We shouldn't assume that Linux is always case-sensitive.
- Don't include case-sensitive filename in tree
Was used for tests, but this broke channel updates because there's a
check to make sure there's no case-sensitive files!
https://hydra.nixos.org/build/233371356/nixlog/1
