Add advisorypkg object.
Add advisorypkglist object.
Add hy_advisory_get_packages method.
tests: add tests for advisorypkg object.
py: add _hawkey.AdvisoryPkg type.
py: add _hawkey.Advisory.packages attribute.
tests: py: add tests for _hawkey.AdvisoryPkg type.
api change: deprecate hy_advisory_get_filenames.
api change: drop deprecated hy_package_get_update_*.
api change: py: deprecate _hawkey.Advisory.filenames.