[9.x] Fix relative links for regular files

[luchaos]

Laravel's storage:link --relative command breaks when configuring paths to regular files.

How to reproduce

Add symfony/filesystem to support relative links:

composer require symfony/filesystem

Add link to regular file in config/filesystems.php:

'links' => [
    public_path('README.md') => base_path('README.md'),
],

Run storage link command with relative option:

php artisan storage:link --relative

Results in...

  ErrorException 

  symlink(): No such file or directory

... because the relative path now has a trailing slash: ../README.md/.

This PR mitigates a bug in Symfony's filesystem component which dates back all the way to 2012, introduced in Laravel 8.x.

I wouldn't expect this bug to be fixed in Symfony itself anytime soon given its history. The fix on Laravel's framework level is simple enough without breaking any existing behaviour (I don't think this particular case ever worked in the first place).

This could be backported to 8.x easily if desired; adhering to the support policy.