Merge (or so) generic phabricator related stuff in a dedicated package between snippets/swh-team and pyarcanist

especially https://forge.softwareheritage.org/source/snippets/browse/master/swh-team/swhphab.py and https://forge.softwareheritage.org/source/pyarcanist/ should be merges / based on a common code base.

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Migrated from T1622 (view on Phabricator)