We know we want exactly want output in `DerivationGoal` now (since
recent refactors), so we can start simplifying things to take advantage
of this.

We don't need the `wanted` field. Just inline the other two fields.

More taking advantage of single wanted output. Also `auto *` not `auto`
for easy reading.

Now that the loops is gone, we can just inline this mutation to a single
simple expression.

`Store::queryPartialDerivationOutputMap` is nothing but checking
statically-known output paths, and then `Store::queryRealisation`, and
we were doing both of those things already. Inline that and simplify,
again taking advantage of the fact that we only care about one output.

Now that `DerivationGoal::checkPathValidity` is legible, we can see that
it only sets `outputKnown`, and doesn't read it. Likewise, with
co-routines, we don't have tiny scopes that make local variables
difficult. Between these two things, we can simply have
`checkPathValidity` return what it finds, rather than mutate some state,
and update everyting to use local variables.

The same transformation could probably be done to the other derivation
goal types (which currently, unfortunately, contain their own
`checkPathValidity`s, though they are diverging, and we hope and believe
that they continue to diverge).

We don't need it any more, because we only used it in the
single-wanted-output `DerivationGoal`.

We can shuffle around control flow so it's only called once. You'll
definitely want to review this diff ignoring whitespace.

Move output result filtering logic and assert just into the branch where
it is not obviously a no op / meeting the assertion.

Add a comment too, while we are at it.

We don't need to ask all these callers to build these single-entry maps
for us.

We can set both during construction, yay!