solver3: Discover recursive dependencies

When we have discovered all clauses for a version, discover
each possible solution for the clauses. This means that when
Discover(foo) is called _anything_ that could lead to foo becoming
uninstallable is translated; so we can extend this next by keeping
a list of reverse dependencies for each package and rejecting
those.

We limit the discovery to those variables that we did not already
enqueue as a negative fact at the root level, as those can never
become true.

We are utilizing a queue here which is not the most performant
solution possible, but where it excels is in producing usable
stack traces when debugging. Traversing the entire dependency
tree using recursion can easily produce thousand levels of
recursion.

The queue means that we discover packages in a breadth-first
manner compatible with the order in which we propagate dependencies,
which is helpful for consistency.

The queue did not appear as a bottleneck in benchmarking. If it did,
we could switch to a grow-only ring buffer (std::queue's underlying
deque also shrinks automatically which is suboptimal).

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