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/test/test.pb.go b/internal/testprotos/test/test.pb.go
index 91ac82e..cbf1c48 100644
--- a/internal/testprotos/test/test.pb.go
+++ b/internal/testprotos/test/test.pb.go
@@ -3314,7 +3314,7 @@
 }
 
 var file_test_test_proto_enumTypes = make([]protoreflect.EnumType, 4)
-var file_test_test_proto_msgTypes = make([]protoimpl.MessageType, 37)
+var file_test_test_proto_msgTypes = make([]protoimpl.MessageInfo, 37)
 var file_test_test_proto_goTypes = []interface{}{
 	(ForeignEnum)(0),                          // 0: goproto.proto.test.ForeignEnum
 	(TestReservedEnumFields)(0),               // 1: goproto.proto.test.TestReservedEnumFields