make project data via API report forks of this project

I want to get a project, via the API, and know all of the forks that exist, or at least those that I can access. This information is not reported. The API reports the forked-from of a project, but not the reverse.

The next step would be to get all accessible projects and look at their forked_from_project.id, but that isn't possible right now because of #19361 (closed). Even so, it's not great, as it may require multiple GETs because of pagination.

I'd prefer, instead, to get an array of forked_projects, with at least ids, but getting owner would save me quite a few more API calls.