Implement support for repos with modern-style arch:all layout

Hi!

This patch implements support for modern APT repositories which have arch:any and arch:all packages in separate Packages files, as outlined in the Debian repository documentation: https://wiki.debian.org/DebianRepository/Format#Architectures

As far as I can see (after some testing), debootstrap is the only tool in common use that does not support this style of APT repositories yet, and having support for it would be extremely great (it makes building APT repos a lot simpler).

I have tested bootstrapping with a new-style, and old-style and the current Debian repository (mixed-style, supporting both), and the patch worked fine for me.

Thanks for considering!