Part of lp.archiveuploader.uploadpolicy View In Hierarchy
This is, in practice, just a marker interface for us to look up upload policies by name.
If registered as a utility, any classes implementing this must be given as the 'component' argument of the <utility> directive so that a getUtility() call returns the class itself rather than an instance. This is needed because the callsites usually change the policies (based on user-specified arguments).