hide implicit deps in apt-cache again by default

hide implicit deps in apt-cache again by default

Before MultiArch implicits weren't a thing, so they were hidden by
default by definition. Adding them for MultiArch solved many problems,
but having no reliable way of detecting which dependency (and provides)
is implicit or not causes problems everytime we want to output
dependencies without confusing our observers with unneeded
implementation details.

The really notworthy point here is actually that we keep now a better
record of how a dependency came to be so that we can later reason about
it more easily, but that is hidden so deep down in the library internals
that change is more the problems it solves than the change itself.