reflect/protoreflect: rename methods with Type suffix

The protobuf type system uses the word "descriptor" instead of "type".
We should avoid the "type" verbage when we aren't talking about Go types.
The old names are temporarily kept around for compatibility reasons.

Change-Id: Icc99c913528ead011f7a74aa8399d9c5ec6dc56e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172238
Reviewed-by: Herbie Ong <herbie@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/legacy_extension.go b/internal/impl/legacy_extension.go
index 4a60f05..e187127 100644
--- a/internal/impl/legacy_extension.go
+++ b/internal/impl/legacy_extension.go
@@ -149,7 +149,7 @@
 }
 
 func (p legacyExtensionTypes) Register(t pref.ExtensionType) {
-	if p.mi.PBType.FullName() != t.ExtendedType().FullName() {
+	if p.mi.PBType.FullName() != t.Extendee().FullName() {
 		panic("extended type mismatch")
 	}
 	if !p.mi.PBType.ExtensionRanges().Has(t.Number()) {
diff --git a/internal/impl/legacy_test.go b/internal/impl/legacy_test.go
index 84cf071..799289b 100644
--- a/internal/impl/legacy_test.go
+++ b/internal/impl/legacy_test.go
@@ -769,7 +769,7 @@
 								// Ignore New since it a constructor.
 							case "Options":
 								// Ignore descriptor options since protos are not cmperable.
-							case "EnumType", "MessageType", "ExtendedType":
+							case "Oneof", "Extendee", "Enum", "Message":
 								// Avoid descending into a dependency to avoid a cycle.
 								// Just record the full name if available.
 								//
@@ -778,6 +778,12 @@
 								if !v.IsNil() {
 									out[name] = v.Interface().(pref.Descriptor).FullName()
 								}
+							// TODO: Remove this when the methods are deleted.
+							case "OneofType", "ExtendedType", "EnumType", "MessageType":
+								v := m.Call(nil)[0]
+								if !v.IsNil() {
+									out[name] = v.Interface().(pref.Descriptor).FullName()
+								}
 							default:
 								out[name] = m.Call(nil)[0].Interface()
 							}
diff --git a/internal/impl/message.go b/internal/impl/message.go
index c719e47..6e0f904 100644
--- a/internal/impl/message.go
+++ b/internal/impl/message.go
@@ -115,8 +115,8 @@
 		switch {
 		case fd.IsWeak():
 			fi = fieldInfoForWeak(fd, specialByName["XXX_weak"])
-		case fd.OneofType() != nil:
-			fi = fieldInfoForOneof(fd, oneofsByName[fd.OneofType().Name()], oneofWrappersByNumber[fd.Number()])
+		case fd.Oneof() != nil:
+			fi = fieldInfoForOneof(fd, oneofsByName[fd.Oneof().Name()], oneofWrappersByNumber[fd.Number()])
 		case fd.IsMap():
 			fi = fieldInfoForMap(fd, fs)
 		case fd.Cardinality() == pref.Repeated:
diff --git a/internal/impl/message_field.go b/internal/impl/message_field.go
index 581481c..ec29a9b 100644
--- a/internal/impl/message_field.go
+++ b/internal/impl/message_field.go
@@ -98,8 +98,8 @@
 	if ft.Kind() != reflect.Map {
 		panic(fmt.Sprintf("invalid type: got %v, want map kind", ft))
 	}
-	keyConv := pvalue.NewLegacyConverter(ft.Key(), fd.MessageType().Fields().ByNumber(1).Kind(), legacyWrapper)
-	valConv := pvalue.NewLegacyConverter(ft.Elem(), fd.MessageType().Fields().ByNumber(2).Kind(), legacyWrapper)
+	keyConv := pvalue.NewLegacyConverter(ft.Key(), fd.Message().Fields().ByNumber(1).Kind(), legacyWrapper)
+	valConv := pvalue.NewLegacyConverter(ft.Elem(), fd.Message().Fields().ByNumber(2).Kind(), legacyWrapper)
 	fieldOffset := offsetOf(fs)
 	// TODO: Implement unsafe fast path?
 	return fieldInfo{