eipp: implement version 0.1 of the protocol

The very first step in introducing the "external installation planer
protocol" (short: EIPP) as part of my GSoC2016 project.

The description reads: APT-based tools like apt-get, aptitude, synaptic,
… work with the user to figure out how their system should look like
after they are done installing/removing packages and their dependencies.
The actual installation/removal of packages is done by dpkg with the
constrain that dependencies must be fulfilled at any point in time (e.g.
to run maintainer scripts).

Historically APT has a super micro-management approach to this task
which hasn't aged that well over the years mostly ignoring changes in
dpkg and growing into an unmaintainable mess hardly anyone can debug and
everyone fears to touch – especially as more and more requirements are
tacked onto it like handling cycles and triggers, dealing with
"important" packages first, package sources on removable media, touch
minimal groups to be able to interrupt the process if needed (e.g.
unattended-upgrades) which not only sky-rocket complexity but also can
be mutually exclusive as you e.g. can't have minimal groups and minimal
trigger executions at the same time.