- For the files that should only occur in the toplevel directory, make sure to
  only match them in the toplevel directory.  This allows having them in an
  imported header only library in some subdirectory.
  + For files such as `.cc` or `.hh` that are matched by extension, there is
    no sane generic way to not check them when they are imported from
    somewhere else, so you'll still have to do something special, sorry.
- For files that are matched by name, make sure that name matches the full
  basename of the file, not just a suffix.
- Pass the regular expressions to perl in a safe way so they can contain
  arbitrary character without arcane quoting.  This mostly applies to the path
  separator '/'.