fix syntax of generated Sources.gz files (Closes: #920227)

The rewrite of dpkg-scan* performed to fix #909847 introduced a problem in 18f42361: Sources.gz entries are not correctly separated by a newline. Furthermore, they have Source: entries instead of Package:

This confuses older version of APT (previous to jessie) which have extra sanity checks on the contents of those files, which breaks building in older chroots.