[6.x] Delete existing links that are broken

[pgrenaud]

This PR attempt to fix an issue that occur when the storage link already exists but is somehow broken. By "broken", I mean link that point to a non-existing target.

When running the storage:link command, we get an error message and the link is not updated.

$ php artisan storage:link

In Filesystem.php line 263:
                                        
  symlink(): No such file or directory  
                                        

To fix this issue, we add an additional check to detect and delete broken links when running the storage:link command.

Implementation details

When a link exists but is broken, file_exists($link) return false. And when symlink($link, $target) is called on a broken link, a PHP Warning is returned and the link is not updated. To fix this, we add an additional check using is_link($link) (which return true, even if the link is broken) to detect and delete broken links.