Incremental copying of binaries doesn't work due to lack of explicit series/pocket selection

The package copier script supports an incremental mode whereby you can copy a source before all its builds have completed (at least in some circumstances), and re-copy it later with additional completed builds. Unfortunately, this feature is inaccessible in practice, because Archive.copyPackage and the PackageCopyJob code don't support specifying the source distroseries or pocket, so you end up copying from the target of the first copy and thus doing nothing of any use.

Adding a facility to specify the source distroseries and pocket should let us do incremental binary copies, which will save us from occasional bizarre manual recovery steps such as those we had to do with KDE and linux early this month, and should make it a lot easier to implement automatic migration from -proposed to release pockets.