add support for "includes"

We have some already existing package definitions which only need to get added to "includes" of the resulting packages.json
Is there a way to archive this?

Start by examining how existing package definitions become the resulting packages.json and how the issue's requested "includes" entries would fit that process. Confirm the expected input and output format with the repository's current behavior; done means existing definitions appear in packages.json under "includes" without breaking other package data.

Written by the indexing model from the issue text.