refact: clear declaration

Author: yu-croco

compiler/src/dotty/tools/io/Directory.scala

@@ -59,7 +59,7 @@ class Directory(jpath: JPath) extends Path(jpath) {
   def files: Iterator[File] = list collect { case x: File => x }
 
   override def walkFilter(cond: Path => Boolean): Iterator[Path] =
-    list filter cond flatMap (_ walkFilter cond)
+    list.filter(cond).flatMap(_.walkFilter(cond))
 
   def deepFiles: Iterator[File] = Path.onlyFiles(deepList())