Code review comment for ~waveform/ubuntu-dev-tools:built-before

The reason for get_sources() unorthodox implementation was simply performance; doko suggested parsing Sources.{gz,xz,etc} directly as otherwise things were liable to either take an insane amount of time or (as I discovered when trying the traditional approach) exceed various limits / timeouts and fail anyway.

I could certainly stuff most of the guts of it into ubuntutools.archive (which is probably the natural home for most of it) and leave the script to do the front-end stuff, if that's preferred? That said .archive is already pretty huge ... maybe a new sub-module would be better; .sources perhaps?