How to develop Magento 2 composer packages

gistfile1.txt

1) Create a folder with your extension.
E.g. extensions/Snippet/YourPackageName
The folder YourPackageName contains your module which has composer and git inited.

2) Execute
composer config repositories.yourname path ../extensions/Snippet/YourPackageName

3) Execute
composer config minimum-stability dev

4) Execute
composer require {your-vendor}/{your-magento-package-name}

Done! Composer will create a symlink to the specified folder.
Now you can develop your extension without executing "composer update package-name" all the time.

P.S.
- you will need to add your repository to "Version Control" in PhpStorm.
Here you will find how to do that: 
https://www.jetbrains.com/help/phpstorm/enabling-version-control.html#associate_directory_with_VCS

- if you face the following exception:
Exception #0 (Magento\Framework\Exception\ValidatorException): Path "/var/www/extensions/Snippet/Controller/view/frontend/templates/controller_list.phtml" cannot be used with directory "/var/www/source/"
try this:
https://magento.stackexchange.com/questions/95071/magento-2-does-not-allow-linking-modules-using-symlinks/