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.