Due to the check soon depending on the base branch (see `--base`),
the CI check can't reasonably share all code with a local check.
We can still make a script to run it locally, just not sharing all code.
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
