[META] Use GitLabCI for packaging

This is not something that we can really prioritize thinking about until issues are migrated.

#3005229: Provide optional support for using composer.json for dependency metadata → would help make the .info.yml rewriting in packaging less-necessary. If we can drop that rewrite, packaging contrib projects becomes significantly more straightforward.

Packagist.org only ships what’s in the repository for a project, see https://github.com/composer/packagist/issues/903

To get more in line with “standard” PHP overall, I expect we’ll want to do less packaging, not more. Even the current minimal packaging can cause annoyances 📌 Packaging info from .info.yml often creates conflicts when patching (ddo) Active

Given the data from #3 (I was under the impression packagist would use releases) it seems reasonable to won't-fix this and instead find (or create) a separate meta to remove packaging customization.