The previous behavior of targets was that targeting a particular node
would implicitly target everything it depends on. This makes sense when
the dependencies in question are between resources, since we need to
make sure all of a resource's dependencies are in place before we can
create or update it.
However, it had the undesirable side-effect that targeting a resource
would _exclude_ any outputs referring to it, since the dependency edge
goes from output to resource. This then causes the output to be "stale",
which is problematic when outputs are being consumed by downstream
configs using terraform_remote_state.
GraphNodeTargetDownstream allows nodes to opt-in to a new behavior where
they can be targeted by _inverted_ dependency edges. That is, it allows
outputs to be considered targeted if anything they directly depend on
is targeted.
This is different than the implied targeting behavior in the other
direction because transitive dependencies are not considered unless the
intermediate nodes themselves have TargetDownstream. This means that
an output1→output2→resource chain can implicitly target both outputs, but
an output→resource1→resource2 chain _won't_ target the output if only
resource2 is targeted.
This behavior creates a scenario where an output can be visited before
all of its dependencies are ready, since it may have a mixture of both
targeted and untargeted dependencies. This is fine for outputs because
they silently ignore any errors encountered during interpolation anyway,
but other hypothetical future implementers of this interface may need to
be more careful.
This fixes#14186.
