Packages: reduce the number of redundant requests

Created by: olafurpg

Currently, we send a request to the package host for every single tracked dependency version, every single time we sync the code host. We should only have to send a request once per version to fetch the dependency and after that we don't have to send more requests for that version. In the rare case that the package host has deleted the package or re-published the version with new content then it's OK to request the site-admin to manually delete the package from our gitserver instance.