Implementing lowpkg module for yum/rpm

Tagging @archtaku, @valintinr.

This is the first step towards decoupling yum and rpm. The same needs to happen for apt and deb. 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.