Support warn_unused_configs in incremental mode #7998

Imagine a config file like this:

[mypy-package.lib.uitls]
disallow_any_generics = True

There is a typo in the section name so the stricter option is not actually applied. Another (probably more common) situation is when a file is moved to a different place, but the config is not updated.

I think we should make it an error (or warning, similar to unrecognized config option) if there is an unmatched section without * wildcards.