How to develop custom module - Symlink local repository

composer.json

    "repositories": [
        {
            "type": "composer",
            "url": "https://packages.drupal.org/8",
            "exclude": [ "drupal/YOUR_MODULE" ]
        },
        {
            "type": "path",
            "url": "../YOUR_MODULE_GIT_CHECKOUT_DIRECTORY"
        }
    ],
    "minimum-stability": "dev",

1. Use "exclude": [ "drupal/YOUR_MODULE" ] - to avoid installing from public repo
2. Define repository path
3. Checkout your module Git repository (and create a composer.json inside if does not exist yet), example:

{
    "name": "drupal/YOUR_MODULE",
    "type": "drupal-module",
    "minimum-stability": "dev",
    "require": {}
}

4. Use composer require drupal/YOUR_MODULE to symlink module to project