Description:
------------
When a RecursiveIteratorIterator's depth reaches the limit, it still may call its sub-iterator's getChildren(). This manifests as performance degradation when using the below script on a directory with many thousands of files in it.

This can be observed by replacing the iterators with a `FilesystemIterator`, which by default won't recurse anyway. As a result, it's 2 orders of magnitude faster than a RecursiveIteratorIterator with depth 0.

With the target folder containing 30k files (NTFS on a PCIe Gen4 SSD):
- RecursiveDirectoryIterator + maxDepth(0) takes 3.7 seconds
- FilesystemIterator takes 0.03 seconds.

This is most observable on Windows due to Windows' abysmal I/O performance.

Test script:
---------------
Slow script:
<?php


$iterator = new RecursiveDirectoryIterator(sys_get_temp_dir() . '/phpstan/cache/nette.configurator');
$iterator2 = new RecursiveIteratorIterator($iterator);
$iterator2->setMaxDepth(0);
$start = hrtime(true);
foreach($iterator2 as $item){

}
var_dump(number_format(hrtime(true) - $start));

--------
Fast script:
<?php

$iterator2 = new FilesystemIterator(sys_get_temp_dir() . '/phpstan/cache/nette.configurator');
$start = hrtime(true);
foreach($iterator2 as $item){

}
var_dump(number_format(hrtime(true) - $start));


Expected result:
----------------
The two scripts should be somewhere in the same order of magnitude of performance.

Actual result:
--------------
The fast script is more than 100x faster than the slow one.