ListImportedProjects: Stop using deprecated fileTreeTraverser method

Use fileTraverser as recommended. Also replace preOrderTraversal with
depthFirstPreOrder as recommended.

Change-Id: I1c13397e3b123879d9c1f6780da80eb3122005d9

src/main/java/com/googlesource/gerrit/plugins/importer/ListImportedProjects.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/importer/ListImportedProjects.java b/src/main/java/com/googlesource/gerrit/plugins/importer/ListImportedProjects.java
index 2e6379c..1381d08 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/importer/ListImportedProjects.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/importer/ListImportedProjects.java
@@ -64,7 +64,7 @@
     Collection<File> importFiles = new HashSet<>();
     File lockRoot = projects.FS_LAYOUT.getLockRoot();
     Path lockRootPath = lockRoot.toPath();
-    for (File f : Files.fileTreeTraverser().preOrderTraversal(lockRoot)) {
+    for (File f : Files.fileTraverser().depthFirstPreOrder(lockRoot)) {
       if (f.isFile()
           && !f.getName().endsWith(".lock")
           && matches(lockRootPath.relativize(f.toPath()))) {