Load placeholder references after BBS import
What does this MR do and why?
Load placeholder references after BBS import
For user contribution mapping, we should make sure all placeholder references are loaded before finalising the import.
References will be generated in a follow-up MR, this is just a logical slice for reviewability's sake.
Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/466356 Changelog: other
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Specs for this MR, functional changes will be described in a follow-up.