Add support for multiple orig tarballs

Add support for multiple orig tarballs

Structure of Upload.pkg is adapted to handle multiple upstream tarballs.
This also means that the .dak format is modified.

Since the .dak format has been modified, improve the object .dak parser
funtion to be able to read both the new and the old format. When
reading the old format, it auto-converts the data to the new structure.
It uses python's type() introspection to know what format is used.

This allows in-place upgrade even when old .dak are still stored
in various queues.

Several other adaptations are also required in other methods of Upload object,
in process-accepted, in autobuild_queue().

Signed-off-by: Raphaël Hertzog <hertzog@debian.org>