This is done by looking for the previousPackageId, and setting the
previous_code attribute on the TrackingResult object. Both code and
previous_code is then queried, and the package code is updated to code
if it doesn't already match.
Tracking code is now in a separate class, which is chosen by a prefix on
the tracking numbers (prefix:code). By subclassing the TrackingModule
class, new sources can easily be added.