[HttpKernel] Fix "absolute path" when we look to the cache directory

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

We should get ``__DIR__`` instead of ``__DIR__ . '/.'``

I've seen this issue when trying to play with the symfony standard new directory structure