Strip wikimedia/composer-merge-plugin from drupal/core-recommended

[Greg Anderson]

Problem/Motivation

The drupal/drupal project composer.lock file contains a reference to wikimedia/composer-merge-plugin. However, this project is not needed for Composer-managed Drupal sites; only the projects in drupal/core (and dependencies thereof) should be listed in drupal/core-recommended.

Proposed Resolution

Add a feature to list projects to ignore in the drupal/core-recommended builder.

Add wikimedia/composer-merge-plugin to ignored list in drupal/core-recommended.