addrs: AbsResource.UniqueKey distinct from AbsResourceInstance.UniqueKey

The whole point of UniqueKey is to deal with the fact that we have some
distinct address types which have an identical string representation, but
unfortunately that fact caused us to not notice that we'd incorrectly
made AbsResource.UniqueKey return a no-key instance UniqueKey instead of
its own distinct unique key type.

internal/addrs/resource.go

@@ -194,8 +194,12 @@ func (r AbsResource) absMoveableSigil() {
 	// AbsResource is moveable
 }
 
+type absResourceKey string
+
+func (r absResourceKey) uniqueKeySigil() {}
+
 func (r AbsResource) UniqueKey() UniqueKey {
-	return absResourceInstanceKey(r.String())
+	return absResourceKey(r.String())
 }
 
 // AbsResourceInstance is an absolute address for a resource instance under a

internal/addrs/resource_test.go

@@ -215,6 +215,77 @@ func TestAbsResourceInstanceEqual_false(t *testing.T) {
 	}
 }
 
+func TestAbsResourceUniqueKey(t *testing.T) {
+	resourceAddr1 := Resource{
+		Mode: ManagedResourceMode,
+		Type: "a",
+		Name: "b1",
+	}.Absolute(RootModuleInstance)
+	resourceAddr2 := Resource{
+		Mode: ManagedResourceMode,
+		Type: "a",
+		Name: "b2",
+	}.Absolute(RootModuleInstance)
+	resourceAddr3 := Resource{
+		Mode: ManagedResourceMode,
+		Type: "a",
+		Name: "in_module",
+	}.Absolute(RootModuleInstance.Child("boop", NoKey))
+
+	tests := []struct {
+		Reciever  AbsResource
+		Other     UniqueKeyer
+		WantEqual bool
+	}{
+		{
+			resourceAddr1,
+			resourceAddr1,
+			true,
+		},
+		{
+			resourceAddr1,
+			resourceAddr2,
+			false,
+		},
+		{
+			resourceAddr1,
+			resourceAddr3,
+			false,
+		},
+		{
+			resourceAddr3,
+			resourceAddr3,
+			true,
+		},
+		{
+			resourceAddr1,
+			resourceAddr1.Instance(NoKey),
+			false, // no-key instance key is distinct from its resource even though they have the same String result
+		},
+		{
+			resourceAddr1,
+			resourceAddr1.Instance(IntKey(1)),
+			false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("%s matches %T %s?", test.Reciever, test.Other, test.Other), func(t *testing.T) {
+			rKey := test.Reciever.UniqueKey()
+			oKey := test.Other.UniqueKey()
+
+			gotEqual := rKey == oKey
+			if gotEqual != test.WantEqual {
+				t.Errorf(
+					"wrong result\nreceiver: %s\nother:    %s (%T)\ngot:  %t\nwant: %t",
+					test.Reciever, test.Other, test.Other,
+					gotEqual, test.WantEqual,
+				)
+			}
+		})
+	}
+}
+
 func TestConfigResourceEqual_true(t *testing.T) {
 	resources := []ConfigResource{
 		{