There now exists an official endpoint for drupal modules and themes. Core can ship with this endpoint specified in it's root composer.json, enabling users to
composer require drupal/rules to install the rules module, using this endpoint.
Patch the composer.json to add the repository specification
The composer endpoints are targeted at being declared beta on July 6th. This can probably be comitted sometime after that
User interface changes
This adds our repository as a package endpoint, and thus adds features that werent there before.
Data model changes