def package_for(package, extension)
      pack = @packages[extension] && @packages[extension][package]
      pack || not_found(package, extension)
    end
def package_for(package, extension)
      pack = @packages[extension] && @packages[extension][package]
      pack || not_found(package, extension)
    end
Look up a package asset list by name, raising an exception if the
 package has gone missing.