Load ordinary overlays without a main source

internal/engine/source.go

@@ -176,8 +176,7 @@ func Mapping(label string, values map[string]any) Source {
 
 // NoSource selects an absent input for defaults and resolution. It creates no
 // source record and has no main declarations to traverse when imports are enabled.
-// An explicit filesystem context may be supplied; active overlays are not
-// available for absent inputs.
+// An active overlay requires explicit filesystem context.
 func NoSource() Source {
 	return Source{kind: sourceNone}
 }