cli: speedup the `swh` cli command startup time

by moving import statements in functions. Also simplify the main help message to to get rid of the necessity to import swh.core.config.

Related to swh-environment#2575 (closed).

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Migrated from D3922 (view on Phabricator)

Some references in the commit message have been migrated:

• T2575 is now swh-environment#2575 (closed)

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Patch application report for D3922 (id=13782)

Rebasing onto 5cc96ef4...

Current branch diff-target is up to date.

Changes applied before test

commit c0c3a897420ddd47db735ed5b05fccaf59202b36
Author: David Douard <david.douard@sdfa3.org>
Date:   Wed Sep 9 19:05:03 2020 +0200

    cli: speedup the `swh` cli command startup time
    
    by moving import statements in functions. Also simplify the main
    help message to to get rid of the necessity to import swh.core.config.
    
    Related to swh/devel/swh-environment#2575.

See https://jenkins.softwareheritage.org/job/DVAU/job/tests-on-diff/16/ for more details.

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