no method to query for packages with a phased_update_percentage

The Ubuntu SRU team runs a script to automatically set the phased_update_percentage of packages in -updates for releases. However, there is no easy way to find packages currently being phased (one with a non-None value for phased_update_percentage) for a release of Ubuntu. Subsequently, getPublishedSources is first used and then for each published source the published binaries (getPublishedBinaries) are examined to see if phased_update_percentage is not None. When discussing this with Colin he said:

09:21 < cjwatson> do file a bug against LP about this, but I think maybe we should extend Archive.getPublishedBinaries to let you query only the ones with non-None phased_update_percentage?