Make composer installer paths consistent between drupal/drupal and the project templates

Problem/Motivation

In the root composer.json file for drupal/drupal, there is a copy of the installer paths configuration for composer/installers:

        "installer-paths": {
            "core": ["type:drupal-core"],
            "modules/contrib/{$name}": ["type:drupal-module"],
            "profiles/contrib/{$name}": ["type:drupal-profile"],
            "themes/contrib/{$name}": ["type:drupal-theme"],
            "drush/contrib/{$name}": ["type:drupal-drush"],
            "modules/custom/{$name}": ["type:drupal-custom-module"],
            "themes/custom/{$name}": ["type:drupal-custom-theme"]
        }

In contrast, the composer installer paths in composer/Template/LegacyProject/composer.json look like this:

        "installer-paths": {
            "core": ["type:drupal-core"],
            "libraries/{$name}": ["type:drupal-library"],
            "modules/contrib/{$name}": ["type:drupal-module"],
            "profiles/contrib/{$name}": ["type:drupal-profile"],
            "themes/contrib/{$name}": ["type:drupal-theme"],
            "drush/Commands/contrib/{$name}": ["type:drupal-drush"]
        }

The differences are:

• drupal/drupal is missing 'libraries', and has the old (early Drush 8) location for Drush extensions.
• drupal/legacy-project is missing the 'custom' paths, and has the correct location for Drush extensions.

n.b. Early versions of Drush 8 expected Drush extensions to be installed to drush/contrib; Drush 9, on the other hand, need Drush extensions to be in drush/Commands/contrib. Recent versions of Drush 8 (Drush 8.2.2+, March 2019) also accept the Drush 9 location.

Proposed resolution

• Update drush/drush to use the same path for Drush extensions as the template projects are using.
• Add libraries to drush/drush.
• Add custom paths to the template projects.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

The composer installer paths in drupal/drupal were unified with those in the template projects, drush/recommended-project and drush/legacy-project.