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.