Draft: debootsnap: fix --ignore-notfound option

The option would prevent an early exit() when searching for the packages on metasnap but debootsnap would still fail to download and then exit later when trying to download non-existing packages with apt. We now communicate the list of packages that were not found (without fatally quitting thanks to --ignore-notfound) to the function which downloads the packages.

EDIT:

As I was working on the above I found more problems and fixed them while I was at it:

• debootsnap is now an order of magnitude faster for large package sets because it now downloads multiple packages with the same apt call instead of starting apt once per package which has a high start-up cost
• instead of using a tuple to store name/ver/arch use a named tuple and type hints