orphans() {
		const tree = this.obj();
		const map = {};

		Object
			.keys(tree)
			.forEach((dep) => {
				tree[dep].forEach((id) => {
					map[id] = true;
				});
			});

		return Object
			.keys(tree)
			.filter((dep) => !map[dep]);
	}
Return a list of modules that no one is depending on.
@api public
@return {Array}