Publishing packages shouldn't allow versions to go backwards by default

Nothing in the current package publishing workflow (the package_publish workflow, the CopyCollectionItems task, or the debian:suite collection manager) prevents rewinding the version of a package to an earlier version. Implementations of Debian archives traditionally disallow this by default, since apt won't normally downgrade packages on client systems automatically. I'm inclined to say that the debian:suite collection manager should check this, since that would be in line with how we handle other constraints.

There should be an override for this, similar to the may_reuse_versions configuration option for suites, since in some experimental suites it may be perfectly fine to do this. Once we implement some way for users to remove items from suites, explicitly removing the "old" (higher) version of the package before publishing the "new" (lower) version would also be fine.