Returns {URL: (size, checksum)}s stored within file.

def _get_sizes_checksums(checksums_path):
  """Returns {URL: (size, checksum)}s stored within file."""
  checksums = {}
  for line in _read_file(checksums_path).split('\n'):
    if not line:
      continue
    # URL might have spaces inside, but size and checksum will not.
    url, size, checksum = line.rsplit(' ', 2)
    checksums[url] = (int(size), checksum)
  return checksums