Introduce modular composer dependencies

[grayside]

Using the wikimedia/composer-merge-plugin project, we can designate separate composer configuration files to facilitate more modular maintenance.

1. This could be done in gadget, and used to treat Octane as a meta-package instead of using our custom merge logic.
2. We could use this to roll up Pattern Lab dependencies into the project root.
3. Particle could add a composer.json inside the Drupal theme and we could include those as well, allowing Particle to declare it's own dependencies.

In order to make this somewhat manageable, we would explicitly disable recursive merge functionality. Adapting from the usage example in the plugin's README, we might have a config such as:

{
    "require": {
        "wikimedia/composer-merge-plugin": "dev-master"
    },
    "extra": {
        "merge-plugin": {
            "include": [
                "composer.local.json",
                "src/themes/*/composer.json",
                "src/themes/*/apps/drupal/composer.json",
            ],
            "recurse": false,
            "replace": false,
            "ignore-duplicates": false,
            "merge-dev": true,
            "merge-extra": false,
            "merge-extra-deep": false,
            "merge-scripts": false
        }
    }
}

I have temporarily flagged this as a question, because how we approach this touches multiple projects and maintainers. I have already discussed this with Bloom, but not sure if the implications for Particle were clear.