Use COCKPIT_DIR instead of __DIR__

When using cockpit as composer dependancy with symlink in public web directory, `$app->script()` and `$app->style()` functions return full path (`/var/www/project/vendor/aheinze/cockpit/assets/...`), because `__DIR__` and `__FILE__` always contain absolute path with symlinks resolved.

Workaround is to define `COCKPIT_DIR` before bootstrap.php file loaded and use this constant instead of `__DIR__` for resolving paths.