RegexDirectoryIterator can extend RegexIterator

Problem/Motivation

I was working on #3195568: [PP-1] Support recursive discovery for migration plugins and noticed we're using a custom Drupal\Component\FileSystem\RegexDirectoryIterator to iterate over the directory files and apply the regex filter.

It seems the same result can be achieved by using native iterators like this:
new \RegexIterator(new \FilesystemIterator($path), 'pattern');

Actually, that's not true, because the \FilesystemIterator() will iterate over SplFileInfo objects, which returns full path name in toString() method. It makes it harder to set a regular expression for the file name. For example, '/^\d/' pattern won't match 1.yml file.

I think what we can do here - change the base class of RegexDirectoryIterator to simplify the class. Instead of \FilterIterator, we can use \RegexIterator, which already accepts a pattern.

Proposed resolution

Change the base class of RegexDirectoryIterator;

API changes

Nope.