Fix vendor/getinfo on Ubuntu derivatives

Steps to reproduce:

• Configure base-files for an Ubuntu derivative
• Don't add your Ubuntu derivative to apt's vendor/ (in my case this was done to minimize the number of packages with delta, so that we can pull in apt security updates more easily)
• Build apt on a filesystem with non-deterministic readdir() order

Expected result:

• The first loop in getcurrent(), looking for an exact match, doesn't find our derivative
• The second loop in getcurrent(), looking for an ancestor that's neither Debian nor Ubuntu, doesn't find an ancestor for our derivative
• The fallback test for Ubuntu returns "ubuntu"
• My apt depends on ubuntu-archive-keyring

Actual result:

• The first loop behaves as expected
• The second loop behaves as expected
• In my case, the fallback test for Ubuntu returns "tanglu"
• My apt depends on tanglu-archive-keyring, which isn't in my derivative

This can fail for two reasons:

• find(1) doesn't guarantee to list distros in alphabetical order;
• in future there might be a distro that sorts later than ubuntu, although right now there is no such distro

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=924662