drupal 8 include a sandbox module with composer after the move to gitlab

Where i say this needs to be documented better:

https://www.drupal.org/project/project_composer/issues/2772557#comment-13115089

What works:

{
    "require": {
        "dereine/jump_menu": "1.x-dev",
    },
    "repositories": {
        "jump_menu": {
            "type": "package",
            "package": {
                "name": "dereine/jump_menu",
                "version": "1.x-dev",
                "type": "drupal-module",
                "source": {
                    "url": "https://git.drupalcode.org/sandbox/dereine-2747697.git",
                    "type": "git",
                    "reference": "8.x-1.x"
                }
            }
        }
    }
}

With, of course, all the other bits needed to have a Drupal site.