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/cmd/protoc-gen-go/internal_gengo/reflect.go b/cmd/protoc-gen-go/internal_gengo/reflect.go
index 0ff2946..d1dbc03 100644
--- a/cmd/protoc-gen-go/internal_gengo/reflect.go
+++ b/cmd/protoc-gen-go/internal_gengo/reflect.go
@@ -28,7 +28,7 @@
 		g.P("var ", enumTypesVarName(f), " = make([]", protoreflectPackage.Ident("EnumType"), ",", len(f.allEnums), ")")
 	}
 	if len(f.allMessages) > 0 {
-		g.P("var ", messageTypesVarName(f), " = make([]", protoimplPackage.Ident("MessageType"), ",", len(f.allMessages), ")")
+		g.P("var ", messageTypesVarName(f), " = make([]", protoimplPackage.Ident("MessageInfo"), ",", len(f.allMessages), ")")
 	}
 
 	// Generate a unique list of Go types for all declarations and dependencies,