Implementing lowpkg module for apt/deb

Tagging @archtaku, @valintinr.

Mimics yum/rpm functionality added in #3501 and addresses #3473.

This is the first step towards decoupling apt and deb. The same is happening for yum and rpm. The idea is to keep repo management in yumpky.py and apt.py, but move the underlying package management functions into modules that return lowpkg in their virtual function. This is necessary to make package management work on apt-rpm systems, such as ALT Linux.