Implement step 1 in resolver rollout process

As per https://github.com/pypa/pip/issues/8371#issuecomment-652422199:

We need to:

• Add a warning on pip install X for users who are likely to experience problems with the new resolver
• Allow users to turn on the new resolver with --use-feature=2020-resolver
• Remove --unstable-feature=resolver
• Raise an error when a user uses --unstable-feature=resolver - pointing the users to use --use-feature=2020-resolver instead.

UX TODO: @ei8fdb and I can provide the text for the error and warning messages.

[uranusjr]

A simple pip check would suffice, no need to add any flags.

[pfmoore]

Those are the two I can think of. I’ll try to review over the weekend - but this week’s been really busy so I have a lot to catch up on, so I’d say add those two and we can always add extra ones as and when we spot them.