apt::params: Make the class private.

apt::params: Make the class private.

Prevent direct access to apt::params. This will ensure that any other
module cannot blindly access apt::params and get settings that have been
potentially overridden at the apt level.

Our own module still can since any class in apt has a module_name of
'apt' but that's up to us to prevent from happening.

Every setting must now be accessed by a qualified lookup into the apt
namespace.