Contents files, and split of data.tgz/xz

[tcolgate]

Would be nice to generate Contents files for each arch
Another possibility here is that we split the incoming debs into they data and control files and store them in a split form.
This makes generating the contents files quicker and could also save quite a bit of space where package aren't updating beyond the metadata. We could go utterly crazy and actually store the individual files int he blob store, but that would probably be too much