title: How to get the root package path using composer
body: <p>I'm developing a PHP component called <a href="https://github.com/robotdance/php-app-config" rel="noreferrer">php-app-config</a> using <a href="https://getcomposer.org/" rel="noreferrer">composer</a>. 
  This component, once required by another project, and installed using <code>composer install</code>, should look for config files inside the config folder of the <a href="https://getcomposer.org/doc/04-schema.md#root-package" rel="noreferrer">root package</a>, something like <code>root_package/config/config.yml</code>.</p>
  
  <p>The <code>./config/config.yml</code> should exists only in the root package and not inside the component imported by the "require:" in <code>composer.json</code>, as below:</p>
  
  <pre><code>▾ root-package/
    ▸ bin/
    ▸ build/
    ▾ config/
      ▸ locales/
        config.yml
    ▸ src/
    ▸ tests/
    ▾ vendor/
      ▸ composer/
      ▸ phpdocumentor/
      ▸ phpspec/
      ▸ phpunit/
      ▾ robotdance/
        ▾ php-app-config/
          ▾ src/                                                                                                                                                                                               
        Config.php -&gt; how to get the root "config/config.yml" path from here?
          ▸ tests/
        composer.json
        composer.lock
        phpunit.xml
        README.md
  </code></pre>
  
  <p>The root package can be a web app or command line utility.
  Is there any way to get the root package path using composer? If not, what is the better way?</p>