Invalid `use-feature` value breaks pip

Environment

• pip version: 20.2
• Python version: 3.7.7
• OS: Fedora 30

Description I made a bad guess for the syntax of how to enable two pip features permanently, and rendered pip completely broken with a simple pip config set command.

Expected behavior One or both of the following:

• a syntax check when doing pip config set would reject an invalid value
• the invalid configuration value would be a warning and still let pip continue

How to Reproduce

1. Enable a non-existing feature using pip config set global.use-feature <feature>
2. Try to use any pip subcommand (or just pip)
3. Pip crashes with an error message

Output

$ pip config set global.use-feature 2020-resolver,fast-deps
Writing to /home/akaihola/.config/pip/pip.conf
$ pip
An error occurred during configuration: option use-feature: invalid choice: '2020-resolver,fast-deps' (choose from '2020-resolver', 'fast-deps')
$ pip config unset --user global.use-feature
An error occurred during configuration: option use-feature: invalid choice: '2020-resolver,fast-deps' (choose from '2020-resolver', 'fast-deps')

The work-around:

$ rm $HOME/.config/pip/pip.conf

[uranusjr]

You don’t have to use newlines; any space would do (pip just splits with str.split()).

[uranusjr]

I just checked, it seems like --isolated only applies to distutils config files (a historical artifact that I don’t think many ever used), not pip config files.

All pip configuration files are always loaded; you can see the function that gets those files received no flags at all:

https://github.com/pypa/pip/blob/ea10d0e253f497c9fc08097f14cdba6b94b45541/src/pip/_internal/configuration.py#L76-L96

We should have a discussion on this 🙁 But also independent to this issue.