cd0c452 contained a bug where the creation diff for a data resource was
put into a new local variable within the else block rather than into the
diff variable in the parent scope, causing a null diff to always be
produced.
This restores the expected behavior: a computed data resource appears in
the diff, so it can then be fetched during the apply walk.
Previously the "planDestroy" pass would correctly produce a destroy diff,
but the "apply" pass would just ignore it and make a fresh diff, turning
it back into a "create" because data resources are always eager to
refresh.
Now we consider the previous diff when re-diffing during apply and so
we can preserve the plan to destroy and then ultimately actually "destroy"
the data resource (remove from the state) when we get to ReadDataApply.
This ensures that the state is left empty after "terraform destroy";
previously we would leave behind data resource states.
Previously they would get left behind in the state because we had no
support for planning their destruction. Now we'll create a "destroy" plan
and act on it by just producing an empty state on apply, thus ensuring
that the data resources don't get left behind in the state after
everything else is gone.
This implements the main behavior of data resources, including both the
early read in cases where the configuration is non-computed and the split
plan/apply read for cases where full configuration can't be known until
apply time.