Reference Composer Installers as a dependency of a Drupal project

Problem/Motivation

If someone tries to install Drupal core via Composer, it will install it in vendor/drupal/core. To get Drupal installed in the required core folder a custom installer should be provided.

Proposed resolution

Drupal should reference composer/installers as a dependency of a Drupal.

Since it is a dependency of Drupal, not Drupal core, the dependency does not need to be added to the repository and Drupal core makes no assumptions of how it will be installed.

Only when someone executes composer update (from the root of Drupal) will the 3rd-party library be installed. If they never run that command, they'll never get the library.

Steps to test

Inside your Drupal repository root run:

$ rm -r composer.lock core
$ composer install

Afterwards there should be no changes other than in composer.json.

Note that I (tstoeckler) had to also add the following to the repositories section of composer.json:

    {
      "type": "vcs",
      "url": "git@github.com:composer/installers.git"
    }

But it that seems to be a separate issue that I've had before and seems to be somehow related to my environment.

Beta phase evaluation