copying package with custom upload between releases should copy the unpacked custom upload too

[Colin Watson]

https://wiki.ubuntu.com/ArchiveAdministration#Moving%20Packages%20to%20Updates includes several procedures that must be carried out by an archive administrator using a shell when copying packages from -proposed to -updates, in order to copy over the unpacked custom uploads. This is error-prone (we forgot about it when copying update-manager to maverick-updates during the maverick release rush, and as a result the upgrader had to look at -proposed for a while), and will also impede any eventual attempt to move all archive administration work to the API.

The PackageCopier interface should gain the ability to copy custom uploads to the proper place on the filesystem, without manual intervention.