Accept debian/changelog pointing to other files in the source package

Only debian/changelog symlinks that point outside the source are unsafe.

This removes a false warning about safe links used in snapd 2.24.1+17.04:

WARNING: debian dir or changelog is a symbolic link snapd_2.24.1+17.04.dsc

$ dget -qx http://archive.ubuntu.com/ubuntu/pool/main/s/snapd/snapd_2.24.1+17.04.dsc
$ readlink -f snapd-2.24.1+17.04/debian/changelog
.../snapd-2.24.1+17.04/packaging/ubuntu-16.04/changelog