Add swh.loader.core.nar module and make core loader use it

This also exposes a swh nar cli and adds tests.

$ Usage: swh nar [OPTIONS] DIRECTORY

  Compute NAR hashes on a directory.

Options:
  -x, --exclude-vcs               exclude version control directories
  -H, --hash-algo [sha256|sha1]
  -f, --format-output [hex|base32|base64]
  --debug / --no-debug
  -h, --help                      Show this message and exit.

Refs. swh/meta#4979 (closed)