This hopefully completes the work for #828, alongside oclif/plugin-update#1266. As with that PR, some thought is required on whether this will break things (publishing to the wrong URL) for anybody with Oclif v1 config left lying around.

This is a bit larger, since it refactors some of the logic into an s3Keys helper to make things a bit more manageable. There's tests included though, which means the coverage of this code goes up significantly (which is good, because it's quite difficult to test this otherwise).

I haven't contributed much to Oclif in the past so please check this carefully, I think it's all correct and should preserve the previous behaviour but it's very hard to confirm myself without representative setups to test with.