packaging: Always drop snapshot before committing a new one

When a submit request is pending, submitting a new one will fail.

By always deleting the snapshot package we ensure that all the pending submit requests will get revoked.

This may cause problems when a release commit is landed and a submit request is issues, and shortly thereafter a non-release commit is landed causing the snapshot and the pending submit request to be deleted.

I consider this a better tradeoff than requiring people to manually ensure no request is pending before landing any commit.