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/encoding/pack/pack.go b/internal/encoding/pack/pack.go
index 93dc4ed..6cb9aa2 100644
--- a/internal/encoding/pack/pack.go
+++ b/internal/encoding/pack/pack.go
@@ -325,7 +325,7 @@
 				kind = fieldDesc.Kind()
 				switch kind {
 				case protoreflect.MessageKind, protoreflect.GroupKind:
-					subDesc = fieldDesc.MessageType()
+					subDesc = fieldDesc.Message()
 					if subDesc == nil || subDesc.IsPlaceholder() {
 						kind = 0
 					}
diff --git a/internal/encoding/tag/tag.go b/internal/encoding/tag/tag.go
index 23c07f5..72499c1 100644
--- a/internal/encoding/tag/tag.go
+++ b/internal/encoding/tag/tag.go
@@ -166,7 +166,7 @@
 		// The name of the FieldDescriptor for a group field is
 		// lowercased. To find the original capitalization, we
 		// look in the field's MessageType.
-		name = string(fd.MessageType().Name())
+		name = string(fd.Message().Name())
 	}
 	tag = append(tag, "name="+name)
 	if jsonName := fd.JSONName(); jsonName != "" && jsonName != name {
@@ -175,13 +175,13 @@
 	// The previous implementation does not tag extension fields as proto3,
 	// even when the field is defined in a proto3 file. Match that behavior
 	// for consistency.
-	if fd.Syntax() == pref.Proto3 && fd.ExtendedType() == nil {
+	if fd.Syntax() == pref.Proto3 && fd.Extendee() == nil {
 		tag = append(tag, "proto3")
 	}
 	if fd.Kind() == pref.EnumKind && enumName != "" {
 		tag = append(tag, "enum="+enumName)
 	}
-	if fd.OneofType() != nil {
+	if fd.Oneof() != nil {
 		tag = append(tag, "oneof")
 	}
 	// This must appear last in the tag, since commas in strings aren't escaped.