function deepReadDirSync(targetDir) {
  const targetDirAbsPath = path.resolve(targetDir);

  if (!fs.existsSync(targetDirAbsPath)) {
    throw new Error(`Cannot read contents of ${targetDirAbsPath}. Directory does not exist.`);
  }

  if (!fs.statSync(targetDirAbsPath).isDirectory()) {
    throw new Error(`Cannot read contents of ${targetDirAbsPath}, because it is not a directory.`);
  }

  // This does the same thing as its containing function, `deepReadDirSync` (except that - purely for convenience - it
  // deals in absolute paths rather than relative ones). We need this to be separate from the outer function to preserve
  // the difference between `targetDirAbsPath` and `currentDirAbsPath`.
  const deepReadCurrentDir = (currentDirAbsPath) => {
    return fs.readdirSync(currentDirAbsPath).reduce((absPaths, itemName) => {
      const itemAbsPath = path.join(currentDirAbsPath, itemName);

      if (fs.statSync(itemAbsPath).isDirectory()) {
        return absPaths.concat(deepReadCurrentDir(itemAbsPath));
      }

      absPaths.push(itemAbsPath);
      return absPaths;
    }, []);
  };

  return deepReadCurrentDir(targetDirAbsPath).map(absPath => path.relative(targetDirAbsPath, absPath));
}
Recursively read the contents of a directory.

@param targetDir Absolute or relative path of the directory to scan. All returned paths will be relative to this
directory.
@returns Array holding all relative paths
@deprecated This function will be removed in the next major version.