Improve and modernize file handling in Harvest module

[dafeder]

Following up on #4276, we should try to eliminate the Drupal\common\Util\DrupalFiles class, since it's only used in Drupal\harvest\Transform\ResourceImporter and ResourceLocalizer service, uses soon-deprecated system_retrieve_file and does not handle file wrappers correctly.

If it's going to be too hard to remove DrupalFiles:

• Refactor DrupalFiles to play nice with vfsStream and also to not use system_retrieve_file().
• Turn Drupal\Tests\harvest\Functional\Transform\ResourceImporterTest into a kernel test.