Returns the library list from the contrib file
@return {Object}
async function() {
      let contrib_json_path = this.getContribFileName();
      if (!await fs.existsAsync(contrib_json_path)) {
        return {
          version: qx.tool.cli.ConfigSchemas.lockfile.version,
          libraries: []
        };
      }
      return qx.tool.compiler.utils.Json.loadJsonAsync(contrib_json_path);
    }