function create_routes_map(manifest_data) {
	/** @type {RoutesMap} */
	const map = new Map();
	for (const route of manifest_data.routes) {
		if (route.leaf) {
			map.set(route.leaf, { route, proxies: { server: null, universal: null } });
		}
	}
	return map;
}
Collect all leafs into a leaf -> route map
@param {import('types').ManifestData} manifest_data