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)