function writeSearchData(methodNames, sourceFilenames) {
    const dataDir = path.join(outdir, 'data');
    fsExtra.mkdirsSync(dataDir);
    fsExtra.writeJsonSync(path.join(dataDir, 'methodNames.json'), sortStrs(methodNames), 'utf8');
    fsExtra.writeJsonSync(path.join(dataDir, 'sourceFiles.json'), sortStrs(sourceFilenames), 'utf8');
}
Prints into <outdir>/data a `methodNames.json` and a `sourceFiles.json`
    JSON file that contains methods and files that can be searched on the
    generated doc site.
    @param {Array<String>} methodNames - A list of method names
    @param {Array<String>} sourceFilenames - A list of source filenames