Cross-project connections outside `.gitlab-ci.yml`

Description

Something about having cross-project triggers in .gitlab-ci.yml bugs me. It makes forks more difficult because the links are hard-coded, and so the fork will try to trigger the downstream pipeline as well. Maybe that problem is just forward links, whereas backwards links might work better. But then a fork of the downstream will re-trigger any time the same upstream projects are updated. That might be a good thing, but might not.

At any rate, I'm wondering if this kind of project-specific configuration belongs in .gitlab-ci.yml at all. Could/should it be outside that file? Putting it in another file is a small step, but has the same problems above. Could/should it be in the web UI configuration? That violates configuration-as-code, but might be better anyway. Or maybe it should be in a separate, version controlled configuration, that is parallel to the main code repo.

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