apt.go: Add ability to target releases
In order to allow packages from backports to be downloaded, the ability
to target a release when running apt-get install is required.
apt.packages now takes the previously defined list of string or a map
of targets and packages to install. When using a map, in order to
install the packages for the base image, the 'default' key should be
default: ["libfoo", "libbar"] buster-backports: ["libbaaz"]