add a way to close bugs on copyPackage completion

It would be nice to have a "bugs" parameter on Archive.copyPackage, such that the script requesting the copy could ask for a bug or bugs to be closed if the copy succeeds.

The two use cases I expect to need this are:

 1) Somebody without upload privileges files a bug asking for a package to be synced from Debian. Somebody with upload privileges sponsors that using syncpackage, which calls Archive.copyPackage. Right now, they have to wait for e-mail before they know whether it's succeeded or not, and then close the bug if it succeeded. That's somewhat error-prone; it would be better if they could just say "close this bug if it works".

 2) Somebody files a bug about a problems which turns out to be closable by syncing a package from Debian. If the Debian change was in response to the Launchpad bug, then the Debian maintainer can put "LP: #nnnnnn" in the changelog and it will be closed (once bug 833736 is fixed). However, if the association between the Debian change and the Launchpad bug was not discovered until later (perhaps the Launchpad bug was filed afterwards), then that isn't enough. While of course it's possible to close such bugs by hand, it would be convenient to have them associated with the sync request.