Composer can now be used to manage Drupal core, modules, themes, PHP dependencies, etc.

Drupal's root composer.json file has been made user editable so that individual projects can specify their own dependencies independent of Drupal core. This allows users to manage Drupal core, modules, themes, PHP Dependencies, etc. from composer.json.

The new composer.json file is currently not a working example. It needs modifications to the repositories, index.php, and needs to use a custom installer. All of this should be resolved when/if #2372815: [meta] Make the <root>/composer.json file a working example lands.

Drupal core's dependencies are managed from core/composer.json. Developers writing patches that upgrade Drupal core dependencies need to change their working directory to core before adding or updating dependencies.

This is a revert of #1805316: Move composer.json to the project root.