Investigate if/how we could improve `swh` cli command startup time

Currently, usage of the swh "master" cli tool suffers from pretty slow loading perfs. Seems it's not so lazy. It would be nice to be able to kick a bit its startup time.

In my (venv-based) dev environment, I get:

time swh 
Usage: swh [OPTIONS] COMMAND [ARGS]...

  Command line interface for Software Heritage.

Options:
  -l, --log-level [NOTSET|DEBUG|INFO|WARNING|ERROR|CRITICAL]
                                  Log level (defaults to INFO).
  --log-config FILENAME           Python yaml logging
                                  configuration file.

  --sentry-dsn TEXT               DSN of the Sentry instance to
                                  report to

  -h, --help                      Show this message and exit.

Notes:
  If both options are present, --log-level will override the
  root logger configuration set in --log-config.

  The --log-config YAML must conform to the
  logging.config.dictConfig schema documented at
  https://docs.python.org/3/library/logging.config.html.

Commands:
  auth              Authenticate Software Heritage users with...
  content-replayer  Software Heritage Objstorage tools.
  dataset           Software Heritage Dataset Tools
  db                Software Heritage database generic tools.
  db-init           Initialise a database for the Software...
  deposit           Deposit main command
  graph             Software Heritage graph tools.
  icinga_plugins    Main command for Icinga plugins
  identify          Compute the Software Heritage persistent...
  indexer           Software Heritage Indexer tools.
  lister            Software Heritage Lister tools.
  loader            Loader cli tools
  objstorage        Software Heritage Objstorage tools.
  scanner           Software Heritage Scanner tools.
  scheduler         Software Heritage Scheduler tools.
  search            Software Heritage Search tools.
  storage           Software Heritage Storage tools.
  vault             Software Heritage Vault tools.

real	0m1.694s
user	0m1.766s
sys	0m0.803s

And

PYTHONPROFILEIMPORTTIME=1 swh  2>&1 | grep 'import time:' | wc -l
2622

I'm not convinced we are obliged to import 2k6+ packages to load most swh cli commands.

---

Migrated from T2575 (view on Phabricator)