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

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