Debug symbol binary packages are published as if they were normal binaries

When Debusine generates indexes for a suite, it currently includes *-dbgsym packages in the normal Packages files. While this technically works, it's unconventional.

Debian publishes these packages in *-debug suites in https://deb.debian.org/debian-debug/, while IIRC Launchpad typically publishes them in a debug subcomponent (e.g. dists/{suite}/{component}/debug/binary-{architecture}/Packages). I think the latter would make more sense for us, since it generalizes well to large numbers of archives.

(We need similar logic to put udebs in a debian-installer subcomponent too. Might as well do them both at the same time.)