[code-quality] Add dirname() concat to absolute path

[TomasVotruba]

The oldschool dirname() was use to traverse one level up.

This has few downsides:

• the IDE file link does not always (depends on combination of .. and dirname()
• the full absolute path is clearly readable by human
• PHPStan custom rule support works better with easy to read strings

-dirname(__DIR__) . '/vendor/autoload.php';
+__DIR__ . '/../vendor/autoload.php';

[samsonasik]

For note: this needs DIRECTORY_SEPARATOR so it will match with operating system used.

-dirname(__DIR__)
+__DIR__ . DIRECTORY_SEPARATOR . '..'

[TomasVotruba]

I'll use per-os separation intead. The DS is too verbose and it would make code harder to read.
Goal of this rule is straightfoward path 👍