internal/filetypes: fix test breakage

missing go generate

Change-Id: Ic1e7f30ac9e3df1f246ef40921e5833df2a63473
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8365
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/internal/filetypes/types.go b/internal/filetypes/types.go
index 023d78c..ec5e6db 100644
--- a/internal/filetypes/types.go
+++ b/internal/filetypes/types.go
@@ -26,7 +26,7 @@
 // cuegenMake is called in the init phase to initialize CUE values for
 // validation functions.
 func cuegenMake(name string, x interface{}) cue.Value {
-	f, err := cuegenInstance.LookupField(name)
+	f, err := cuegenInstance.Value().FieldByName(name, true)
 	if err != nil {
 		panic(fmt.Errorf("could not find type %q in instance", name))
 	}