Don't require specifying unstable_sources, unstable_packages

My host OS is Ubuntu but I wanted to do rebuilds on packages for Unstable. In my particular case (jpeg-xl transition), there was one Debian source package (wpewebkit) that does not exist in Ubuntu.

I ended up downloading these two files to my system and unxz'd them and updated my config.ini to point to the directory they were in. The rest of the instructions worked for me.

https://ftp.debian.org/debian/dists/sid/main/source/Sources.xz

https://ftp.debian.org/debian/dists/sid/main/binary-amd64/Packages.xz

It would be nice if this project could handle that automatically instead.