internal/impl: rename MessageType as MessageInfo

The name MessageType is easily confused with protoreflect.MessageType.
Rename it as MessageInfo, which follows the pattern set by v1,
where the equivalent data structure is called InternalMessageInfo.

Change-Id: I535956e1f7c6e9b07e9585e889d5e93388d0d2ce
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/178478
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/testprotos/test3/test.pb.go b/internal/testprotos/test3/test.pb.go
index 31180d0..88f664f 100644
--- a/internal/testprotos/test3/test.pb.go
+++ b/internal/testprotos/test3/test.pb.go
@@ -1271,7 +1271,7 @@
 }
 
 var file_test3_test_proto_enumTypes = make([]protoreflect.EnumType, 2)
-var file_test3_test_proto_msgTypes = make([]protoimpl.MessageType, 20)
+var file_test3_test_proto_msgTypes = make([]protoimpl.MessageInfo, 20)
 var file_test3_test_proto_goTypes = []interface{}{
 	(ForeignEnum)(0),                   // 0: goproto.proto.test3.ForeignEnum
 	(TestAllTypes_NestedEnum)(0),       // 1: goproto.proto.test3.TestAllTypes.NestedEnum
diff --git a/internal/testprotos/test3/test_import.pb.go b/internal/testprotos/test3/test_import.pb.go
index a281989..c36e2be 100644
--- a/internal/testprotos/test3/test_import.pb.go
+++ b/internal/testprotos/test3/test_import.pb.go
@@ -114,7 +114,7 @@
 }
 
 var file_test3_test_import_proto_enumTypes = make([]protoreflect.EnumType, 1)
-var file_test3_test_import_proto_msgTypes = make([]protoimpl.MessageType, 1)
+var file_test3_test_import_proto_msgTypes = make([]protoimpl.MessageInfo, 1)
 var file_test3_test_import_proto_goTypes = []interface{}{
 	(ImportEnum)(0),       // 0: goproto.proto.test3.ImportEnum
 	(*ImportMessage)(nil), // 1: goproto.proto.test3.ImportMessage