mercurial.cli: Deprecate cli in favor of the generic swh loader run

This module is:

• not referenced in the setup.py (so no simple cli call)
• not tested
• using an in-memory storage
• no longer the canonical way of triggering a mercurial ingestion

Note that this also fixes the visit date cli input. When user provided, this date should
be parsed as a datetime prior to being passed to the loader constructor.