eipp: implement Immediate-Configuration flag

eipp: implement Immediate-Configuration flag

APT has 3 modes: no immediate configuration, all packages are configured
immediately and its default mode of configuring essentials and
pseudo-essentials immediately only. While this seems like a job of
different planers at first, it might be handy to have it as an option,
too, in case a planer (like apts internal one) supports different modes
where the introduction of individual planers would be counter intuitive.