core: Fix TestContext2Apply_dataBasic

In the reorganization of the data source read code we missed the fact that
the plan phase must _always_ generate a read data diff, never directly
read, because the state generated during plan is throwaway.

This only matters in the -refresh=false case, since normally refresh has
already taken care of this, but that is still an important case, covered
by the TestContext2Apply_dataBasic test.

terraform/eval_read_data.go

@@ -32,6 +32,13 @@ type EvalReadData struct {
 	// in this planned change.
 	Planned **plans.ResourceInstanceChange
 
+	// ForcePlanRead, if true, overrides the usual behavior of immediately
+	// reading from the data source where possible, instead forcing us to
+	// _always_ generate a plan. This is used during the plan walk, since we
+	// mustn't actually apply anything there. (The resulting state doesn't
+	// get persisted)
+	ForcePlanRead bool
+
 	// The result from this EvalNode has a few different possibilities
 	// depending on the input:
 	// - If Planned is nil then we assume we're aiming to _produce_ the plan,
@@ -101,7 +108,7 @@ func (n *EvalReadData) Eval(ctx EvalContext) (interface{}, error) {
 	// If our configuration contains any unknown values then we must defer the
 	// read to the apply phase by producing a "Read" change for this resource,
 	// and a placeholder value for it in the state.
-	if !configVal.IsWhollyKnown() {
+	if n.ForcePlanRead || !configVal.IsWhollyKnown() {
 		// If the configuration is still unknown when we're applying a planned
 		// change then that indicates a bug in Terraform, since we should have
 		// everything resolved by now.

terraform/node_resource_plan_instance.go

@@ -101,6 +101,7 @@ func (n *NodePlannableResourceInstance) evalTreeDataResource(addr addrs.AbsResou
 				Provider:       &provider,
 				ProviderAddr:   n.ResolvedProvider,
 				ProviderSchema: &providerSchema,
+				ForcePlanRead:  true, // _always_ produce a Read change, even if the config seems ready
 				OutputChange:   &change,
 				OutputValue:    &configVal,
 				OutputState:    &state,