Streamline data_files generation with additional keys

Streamline data_files generation with additional keys

* Move data_file generation out of setup().
* Return per-directory keys, since distutils only uses the directory
  key and value filename (not the value path) when installing
  data_files.
* Use relative key paths for more flexible installation.
* Raise NotImplementedError if os.path.sep is not '/', which allows
  for simpler path handling.