Debdiff comparison to sid misses dbgsyms

If you debdiff a package that builds dbgsym, you'll see the -dbgsym packages as being entirely new, because they aren't part of the debian:sid suite (dak splits them out into debian-debug)

e.g. https://debusine.debian.net/debian/r-stefanor-dh-python/artifact/2914218/

It's not completely obvious what the best course of action is here. We could exclude the -dbgsym packages from the diff if the base suite doesn't have them. We could just note the expected difference in the output.