package_manager.py: create separate class for installed packages listing
This commit creates a new class that has the only purpose to generate
various listings of installed packages in the rootfs.
Basically, the methods involved in listing the installed packages, that
were part of each backend PM class implementation, were moved to this
new class.
This change avoids instantiating a new PM object just to get the list of
installed packages in a certain rootfs.
Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
