Validate import_type on project
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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.
issue