refactor: MultiFileProgressMeter.end()'s status shouldn't default to FAILED.
refactor: how download progress are handled.

Introduces special Payload class hierarchy to handle the complexities in
between librepo and display callbacks.
remove: repo.get_package(), get_package_target().

Not used.
refactor: downloads of metadata use the Payload system too.
tests: fix the unit tests after Progress/Payload refactoring.
Turn DRPM into playing by the same Payload rules as RPM.
fup-drpm refactoring
added glob arch pattern support (RhBug:1048788)
fixed path to imported constant
refactored _nevra_to_filters function