Archive:+copy-packages copies from private to public archives without prompting

Archive:+copy-packages is willing to copy from a private to a public archive with no particular extra UI to indicate that this will result in making private files public. This holds both for the old synchronous path (in which case it creates a delayed copy) and the new asynchronous path (in which case it currently oopses due to bug 1026976, but will soon pass the unembargo flag).

It would be safer if there were some kind of additional UI to confirm that this is intentional, much as the asynchronous API deliberately requires an extra flag to enable this.