internal/core/compile: fix hidden definition bug

Fixes #533

Change-Id: Idcefbec32b3321401cf4449375cd194f5f86668f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7265
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/internal/core/adt/feature.go b/internal/core/adt/feature.go
index bc2a32c..74926a6 100644
--- a/internal/core/adt/feature.go
+++ b/internal/core/adt/feature.go
@@ -116,7 +116,7 @@
 	i := r.StringToIndex(s)
 	t := StringLabel
 	switch {
-	case strings.HasPrefix(s, "#_"):
+	case strings.HasPrefix(s, "_#"):
 		t = HiddenDefinitionLabel
 	case strings.HasPrefix(s, "#"):
 		t = DefinitionLabel