See the component page for more information.
Import files are a new invention, so libraries like prettify do not yet provide them.
prettify-import is an intermediary that provides an import file for the prettify component.
prettify-import depends on prettify.
Components that want to use prettify standalone should depend on prettify-import. Such components need not use Polymer or prettify-element, but we put the import and the element in one package for convenience.