Allows detecting whether attributes are overridden in all-packages.nix.
In a future commit we'll use this to detect empty arguments being set in
all-packages.nix and complain about that.
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