Allow a project to be forked to a different shard than it's source

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Since we've introduced object deduplication, we've made it so that every project that is forked gets stored on the same shard as the source project.

Since !21339 (merged) we allow a project to have multiple pool repositories across different shards.

After we keep maintaining those pools (#39086). We could also make sure that we separate the forks across multiple shards to avoid hotspots on the single shard that contains every fork of a popular project.