docs: def format_files(src: str, dest: str, **fmt_vars: str) -> None:
      """Copies all files inside src into dest while formatting the contents
      of the files into the output.
  
      For example, a file with the following contents:
  
      {foo} bar {baz}
  
      and the vars {'foo': 'herp', 'baz': 'derp'}
  
      will end up in the output as
  
      herp bar derp
      :param text src: Source directory.
      :param text dest: Destination directory.
      :param dict fmt_vars: Vars to format into the files.
      """
      assert os.path.exists(src)
      assert os.path.exists(dest)
      # Only at the root.  Could be made more complicated and recursive later
      for filename in os.listdir(src):
          if filename.endswith(EXCLUDED_EXTENSIONS):
              continue
          # Flat directory structure
          elif not os.path.isfile(os.path.join(src, filename)):
              continue
          with open(os.path.join(src, filename)) as f:
              output_contents = f.read().format(**fmt_vars)
          with open(os.path.join(dest, filename), 'w') as file_obj:
              file_obj.write(output_contents)