more explicit MarkRequired algorithm code

more explicit MarkRequired algorithm code

Piling everything in a single if statement always made my head wobble,
but it hasn't even a benefit as the most common case of a package which
isn't installed passes all of the old if and lands in the non-existent
else-part of the inner if. So beside a subjective cleanup of what goes
on this implementation should also be a bit faster.

No change in behavior should be present.

Gbp-Dch: Ignore
(cherry picked from commit 769e9f3ea1cbe67d3b98e6db6c956abde2384868)