PhpStorage\FileStorage tries to write a file using the same name of MTimeProtectedFileStorage's directory name

Problem

• When switching from the default PhpStorage\MTimeProtectedFileStorage to PhpStorage\FileStorage, you get this fatal error:
  Warning: file_put_contents(sites/default/files/php/service_container/service_container_prod_.php): failed to open stream:

Permission denied in Drupal\Component\PhpStorage\FileStorage->save()
(line 60 of core\lib\Drupal\Component\PhpStorage\FileStorage.php).

Drupal\Component\PhpStorage\FileStorage->save('service_container_prod_.php', '...

Cause

• sites/default/files/php/service_container/service_container_prod_.php is a directory.

Proposed solution

1. Change MTimeProtectedFileStorage to not use a .php file suffix for its directory.