Sync can fail if multiple collections share a dependency

[gerrod3]

Version
<=0.17

Describe the bug
Dependency sync with multiple collections sharing a dependency can cause sync failures if the upstream server has rate-limiting. While the shared dependency will only be downloaded once, the metadata for that collection will be fetched multiple times on V2 syncs and can cause a rate-limit. The V2 sync machinery should remember the metadata for collections it has already visited to avoid this problem.