internal/filedesc: use genid constants for map entry fields

Change-Id: If551fc4f3e8da6325af7a0125f0e96fb014d3636
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/238998
Reviewed-by: Herbie Ong <herbie@google.com>

internal/filedesc/desc.go

@@ -13,6 +13,7 @@ import (
 	"google.golang.org/protobuf/internal/descfmt"
 	"google.golang.org/protobuf/internal/descopts"
 	"google.golang.org/protobuf/internal/encoding/defval"
+	"google.golang.org/protobuf/internal/genid"
 	"google.golang.org/protobuf/internal/pragma"
 	"google.golang.org/protobuf/internal/strs"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
@@ -302,13 +303,13 @@ func (fd *Field) MapKey() pref.FieldDescriptor {
 	if !fd.IsMap() {
 		return nil
 	}
-	return fd.Message().Fields().ByNumber(1)
+	return fd.Message().Fields().ByNumber(genid.MapEntry_Key_field_number)
 }
 func (fd *Field) MapValue() pref.FieldDescriptor {
 	if !fd.IsMap() {
 		return nil
 	}
-	return fd.Message().Fields().ByNumber(2)
+	return fd.Message().Fields().ByNumber(genid.MapEntry_Value_field_number)
 }
 func (fd *Field) HasDefault() bool                           { return fd.L1.Default.has }
 func (fd *Field) Default() pref.Value                        { return fd.L1.Default.get(fd) }