Allow absent inputs to use import contexts

internal/engine/source.go

@@ -174,7 +174,10 @@ func Mapping(label string, values map[string]any) Source {
 	return source
 }
 
-// NoSource explicitly selects a defaults-only input. It creates no source record.
+// 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.
 func NoSource() Source {
 	return Source{kind: sourceNone}
 }