code: function resolveExternalPath(importPath, options) {
    let baseIndex = importPath[0] === '@' ? importPath.indexOf('/') + 1 : 0;
    let addonName = importPath.substring(0, importPath.indexOf('/', baseIndex));
    let addon = options.parent.addons.find(addon => addon.name === addonName);
  
    if (!addon) {
      throw new Error(`Unable to resolve styles from addon ${addonName}; is it installed?`);
    }
  
    let pathWithinAddon = importPath.substring(addonName.length + 1);
    let addonTreePath = path.join(addon.root, addon.treePaths.addon);
  
    let absolutePath = ensurePosixPath(path.resolve(addonTreePath, pathWithinAddon));
    let keyPath = options.addonModulesRoot + addonName + '/' + pathWithinAddon;
    return new DependencyPath('external', absolutePath, keyPath);
  }
docs: Resolve absolute paths pointing to external addons