sync reports have an error when diffing source repos

[tfoote]

The exception looks like this:

Exeception: Failed to load from url file:///var/repos/ubuntu/main/dists/xenial/main/source/Packages [<urlopen error [Errno 2] No such file or directory: '/var/repos/ubuntu/main/dists/xenial/main/source/Packages'>] 

This is because source directories differ from binary directories in the debian repository layout and use Source.gz instead of Packages for the filename.

The correct thing to do would be to detect we're looking for source entires, switch the url to Source.gz and unpack the result before trying to diff the packages.

Right now it's the exception is caught and ignored. And we don't need the result from that section. So this is a low priority enhancement.

Thanks to @wjwwood for flagging this. An example of this failure is: http://build.ros.org/job/Jrel_sync-packages-to-main/12/consoleFull