this would allow a composer.json file at /alternative/location/composer.json be configured with

  "extra": {
    "runtime": {
      "project_dir": "/../../"
    }
  }

while using

COMPOSER=/alternative/location/composer.json composer update

generate a autoload_runtime.php with the following project_dir

$runtime = new $runtime(($_SERVER['APP_RUNTIME_OPTIONS'] ?? $_ENV['APP_RUNTIME_OPTIONS'] ?? []) + [
  'project_dir' => dirname(__DIR__, 1),
]);

i did not meddle with the autoload template discovery as that's currently already configurable and would introduce a bc break. also it makes sense in a way that every config value is relative to the composer.json file itself