Laravel config file of a module

I need to get the keys of a file inside the config of a module. I created User module using nWidart package. This is the registerConfig function

protected function registerConfig()
{
    $this->publishes([
        __DIR__.'/../Config/config.php' => config_path('user.php'),
    ], 'config');
    $this->mergeConfigFrom(
        __DIR__.'/../Config/config.php', 'user'
    );
}

I created a file named menu.php that returns an array with a key items, then tried this

dd(config('user::config.name'))
dd(config('user::menu.items'))

They both return null. I also tried php artisan vendor-publish of UserServiceProvider but it still returns null. How can I return the keys?

update

If I simply do: config() I get a full array of the config arrays and can see my package config file there. But the menu config file is not in there

In Your serviceProvider, register method.

    $this->mergeConfigFrom(
        __DIR__ . '/../config/master.php',
        'master'
    );

    // Merge configs
    $this->mergeConfigFrom(
        __DIR__ . '/../config/slave.php',
        'slave'
    );

Create two files inside config folder slave.php

    <?php
return [
    'menu' => [
        'OP1',
        'OP2',
        'OP3',
        'OP4',
    ]
];

master.php //any other config you want to load

Run php artisan config:cache

and use

dd(config('slave.menu')); dd(config('master.XXX'));

This will output

array:4 [▼
  0 => "OP1"
  1 => "OP2"
  2 => "OP3"
  3 => "OP4"
]