Loading dependencies from the vendor directory in a module using composer
Composer Manager is just a small wrapper around the Merge Plugin which we have in core nowadays. 

Register your module in the root composer.json of your project, look for extra > merge-plugin > include. It should look like this:

"extra": {
    "_readme": [
        "By default Drupal loads the autoloader from ./vendor/autoload.php.",
        "To change the autoloader you can edit ./autoload.php."
    ],
    "merge-plugin": {
        "include": [
            "core/composer.json",
            "modules/facebook_ratings/composer.json"
        ],
        "recurse": false,
        "replace": false,
        "merge-extra": false
    }
}


modules/facebook_ratings/composer.json is the path to your composer.json. It depends on where you have your module.

Run composer install afterwards.