_getModulesRootFolder() {
    if (this.config.modulesFolder) {
      return this.config.modulesFolder;
    } else if (this.config.workspaceRootFolder) {
      return this.config.workspaceRootFolder;
    } else {
      return path.join(this.config.lockfileFolder, (_constants || _load_constants()).NODE_MODULES_FOLDER);
    }
  }
Get the common ancestor of every node_modules - it may be a node_modules directory itself, but isn't required to.