Client/Tools/APT: remove all the stuff for the old API
Revert "APT: add backwards compatibility for python-apt 0.6"

This reverts commit 81cc9e874928ee97283cbf501b4ffd3917f1f476.
Client/Tools/APT: use "pkg in cache" everywhere

The "in syntax" is supported instead of the cache.has_key(pkg) syntax
at least since python-apt-0.7.91.
Client/Tools/APT: use more modern python-apt API

python-apt provides a public API for all required steps.
Client/Tools/APT: some syntax cleanup