static _walk(dirPath, callback) {
    const entries = fs.readdirSync(dirPath);

    for (const entry of entries) {
      const entryPath = path.resolve(dirPath, entry);
      const stat = fs.statSync(entryPath);

      if (stat.isFile()) {
        callback(entryPath);
      } else if (stat.isDirectory()) {
        this._walk(entryPath, callback);
      }
    }
  }
walk recursive in directory.
@param {string} dirPath - target directory path.
@param {function(entryPath: string)} callback - callback for find file.
@private