App.distro_id used to return "opensuse-tumbleweed", analogous to
what's in ID@/etc/os-release, but now returns "opensuse", and the
"tumbleweed" goes into "codename". That matches more what Debian-like
distributions do, but it confuses _backend_path. Adapt it to map the
new distro_id correctly.
Signed-off-by: Jan Lindemann <jan@janware.com>