code: function convertExternalDependencyDefinition( def ) {
    if ( typeof def === 'string' ) {
      const defAsString = {};
      defAsString[ def ] = "";
      return defAsString
    }
    if ( Array.isArray( def ) ) {
      const defAsArray = {};
      def.forEach( function ( file ) {
        defAsString[ file ] = "";
      } );
    }
    return def;
  }
docs: There are three ways to define a list of external dependencies:
  * "foo.js"
  * ["foo.js", "bar.js"]
  * { "foo.js": "libs/foo.js", ... }
  
  This function transforms a definition in its third form: `{ "foo.js": "libs/foo.js", ... }`.
  
  @param   {any} def - Definition of the dependencies.
  @returns {object} Definition in the third form.