Rework script which checks for uninstallable packages

While older versions of dpkg reported:

| No packages found matching ${package}.

newer ones report:

| dpkg-query: no packages found matching ${package}

Newer versions of dpkg also seem to mix stdout and stderr
(looking like buffering problems?), so the result is unreliable
for us. Therefore split handling of stdout and stderr output.

This fixes the problem that the junit output wasn't
really adequate anymore lately.