cli: migrate away from optparse

[oliver-sanders]

superseedes #2972, #3525

optparse has served us well but is a little long in the tooth, we may want to consider moving to a better supported system.

The CLI is now quite a bit simpler than before and our future plans (#3592) aren't too demanding on the option parser so this is now a low priority issue.

Desirable functionality:

• Ability to auto-document with Sphinx - autodocument CLI cylc-doc#185.
• Ability to generate a Python API and CLI from the same src / with the same args/docs.
• Allow efficient Cylc plugins - plugin framework #4106.

We have previously considered other options including:

• argparse
• click
• clize

For context see the discussion on:

• CLI refactor #2972
• unify cylc cli (under click?) #3525

Pull requests welcome!