Follow-up from "Resolve "Move "include external files in .gitlab-ci.yml" from Starter to Libre""

The following discussions from gitlab-ce!21603 should be addressed:

@grzesiek started a discussion: (+2 comments)

I wonder if we should move this module to Config. Is that external CI or external CI config?
@grzesiek started a discussion:

I sometimes wonder if this code should belong to Config::Loader or perhaps we should have another class to combine multiple configs. This does not need to improved / resolved in this merge request, of course, I'm just wondering / thinking about it right now 😸