Use apt-cache to pre-filter PackageKit queries

If some of the packages passed to PackageKit don't exist, it may fail to
report on some of the other packages you asked about too. So make sure
they all exist first.

Also, apt-cache support was broken anyway because apt-cache no longer
accepts the --no-all-versions option after the subcommand name.

When running in debug mode, don't redirect apt-cache's stderr to
/dev/null.