Support regular composer-based installation

Currently the modules in cfr package require composer_manager to integrate 3rd party libraries.

• Enabling modules causes composer.json to be rebuilt and new packages to be downloaded.

The modern way to use Composer with Drupal 7 is to do something like https://github.com/drupal-composer/drupal-project

• use Composer to download modules and 3rd party packages.
• enabling or disabling modules does not change the composer packages.

The 7.x-2.x branch should use this superior way of integrating with Composer.

Roadmap:

1. Release a 7.x-2.x version which works cleanly with drupal-composer/drupal-project.
2. Provide BC support to allow the 7.x-2.x version to also work with composer_manager.
3. Provide a migration path from composer_manager to the new method.