Fix the detection logic for needed package builds

Previously, packages that have different source and binary package names
(eg xulrunner-geckofx) were always rebuilt

Change-Id: I0b18ee0f9b908551372672f34ce8a25c44c7fb3d
Skip distributions that don't exist in pbuilder

Change-Id: Ieb5ec806476ffbb3a416f716f2b07a0f31e1a0a8