When setting prefixes to empty strings, check the key names in addition to value

[hroncok]

In Fedora, we are currently trying to come up with a safe approach that
will make sudo pip and similar install to /usr/local rather than /usr.

After years of only patching distutils for this,
we need to adapt our change to use sysconfig.

In the past, we've tried to use a custom installation scheme,
but that has yielded many unexpected side effects for our users.

Now we try to set the defaults of {base} and {platbase} to /usr/local instead.
However, such value is not in the prefixes listed in this check in virtualenv.

Here, I adapted the code to be more robust for such changes:

• if the value is listed in prefixes, we still set to empty
• if the key is listed in prefix_vars, we now also set to empty

Let me know if this is acceptable to virtualenv. An alternative would be something like:

if hasattr(sysconfig, 'extra_prefixes'):
    prefixes = prefixes + tuple(sysconfig.extra_prefixes)

Or:

if hasattr(sys, 'insatllation_prefix'):
    prefixes = prefixes + (sys.insatllation_prefix,)

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)!

• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

I plan to work on tests and docs after we agree on the best approach to fix this.

[hroncok]

I am unsure if the check / test [email protected] - macos-12 failure is relevant.