LitElement ESM build 1/2 - rewrite lit-element module specifier

rollup.config.js

export default {
  input: 'src/hello-element.js',
  // exclude lit-element dependency 
  external: ['lit-element'],
  output: {
    file: `lib/hello-element.esm.js`,
    // keep ES Module format
    format: 'es',
    // rewrite bare module specifier to a relative path
    paths: {
      'lit-element': './lit-element.js'
    }
  }
};