lib/collect-file-info.js
function directoriesUnder (rootDir) {
  return directoryContents(
    rootDir,
    stats => stats.isDirectory(),
    (path, stats) => path
  )
}
function directoriesUnder (rootDir) {
  return directoryContents(
    rootDir,
    stats => stats.isDirectory(),
    (path, stats) => path
  )
}
collect