Add API to download packages

This is one of the necessary bits previous to implementing offline updates, but would also help for regular updates, since it could potentially reduce the time updates take to execute if things are downloaded before-hand. Ideally, the packages would be temporarily downloaded to /var/cache or equivalent and then used from there instead of re-downloading them if their checksums did not change. I am not sure if libapk has support for this, but if it does not, we would have to start from there :)