Validate import_type on project
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=341533) </details> <!--IssueSummary end--> The following discussion from !70012 should be addressed: - [ ] @dstull started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70012#note_684304395): (+3 comments) > Ideally, we can metaprogrm this if I can change the `github_import` to be `github_importer` - so we'll need to play with this a bit before going into final review.
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