re-add ModuleInstance -> Module conversion

When working with a ConfigResource, the generalization of a
ModuleInstance to a Module was inadvertently dropped, and there was to
test coverage for that type of target.

Ensure we can target a specific module instance alone.

terraform/context_plan_test.go

@@ -6164,3 +6164,50 @@ resource "test_instance" "b" {
 	_, diags := ctx.Plan()
 	assertNoErrors(t, diags)
 }
+
+func TestContext2Plan_targetedModuleInstance(t *testing.T) {
+	m := testModule(t, "plan-targeted")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Config: m,
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("aws"): testProviderFuncFixed(p),
+		},
+		Targets: []addrs.Targetable{
+			addrs.RootModuleInstance.Child("mod", addrs.IntKey(0)),
+		},
+	})
+
+	plan, diags := ctx.Plan()
+	if diags.HasErrors() {
+		t.Fatalf("unexpected errors: %s", diags.Err())
+	}
+	schema := p.GetSchemaReturn.ResourceTypes["aws_instance"]
+	ty := schema.ImpliedType()
+
+	if len(plan.Changes.Resources) != 1 {
+		t.Fatal("expected 1 changes, got", len(plan.Changes.Resources))
+	}
+
+	for _, res := range plan.Changes.Resources {
+		ric, err := res.Decode(ty)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		switch i := ric.Addr.String(); i {
+		case "module.mod[0].aws_instance.foo":
+			if res.Action != plans.Create {
+				t.Fatalf("resource %s should be created", i)
+			}
+			checkVals(t, objectVal(t, schema, map[string]cty.Value{
+				"id":   cty.UnknownVal(cty.String),
+				"num":  cty.NumberIntVal(2),
+				"type": cty.StringVal("aws_instance"),
+			}), ric.After)
+		default:
+			t.Fatal("unknown instance:", i)
+		}
+	}
+}

terraform/testdata/plan-targeted/main.tf

@@ -3,5 +3,10 @@ resource "aws_instance" "foo" {
 }
 
 resource "aws_instance" "bar" {
-    foo = "${aws_instance.foo.num}"
+    foo = aws_instance.foo.num
+}
+
+module "mod" {
+  source = "./mod"
+  count = 1
 }

terraform/testdata/plan-targeted/mod/main.tf

@@ -0,0 +1,3 @@
+resource "aws_instance" "foo" {
+    num = "2"
+}

terraform/transform_targets.go

@@ -145,6 +145,8 @@ func (t *TargetsTransformer) nodeIsTarget(v dag.Vertex, targets []addrs.Targetab
 				targetAddr = target.ContainingResource().Config()
 			case addrs.AbsResource:
 				targetAddr = target.Config()
+			case addrs.ModuleInstance:
+				targetAddr = target.Module()
 			}
 		}