Refactoring: Implement do_package in treepkg.packager.SourcePackager. The actual implementation in the derived classes is almost identical in all cases so it's better to have as much of the implementation in the base class. The update_version_numbers method is not called directly by the base class code so is removed from the base class. OTOH, prepare_sources_for_tarball has been added as a more general variant of update_version_numbers that is actually called by the default implementation of do_package.