Migrate merge request dependencies
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### Problem
Users expect important object relationships to be preserved during project migration.
This should include the various [merge request dependencies](https://docs.gitlab.com/user/project/merge_requests/dependencies/) for MRs.
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They may be added when creating/editing an MR, e.g.
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### Proposed solution
Relations for merge request dependencies should be migrated as part of a project import.
issue