Exclusive Option Groups

Flags can be mutually exclusive, but there isn't a way to do that for options. e.g. you can't have `[--arch | --target ]` Ideally these could be nested like `[[--one --two ] | [--combo ]]`

Start by tracing the existing mutually exclusive flag handling and the option-parsing entry points. Compare their behavior with the proposed exclusive option groups, including nested alternatives such as `[--arch | --target ]` and `[[--one --two ] | [--combo ]]`. Done means option groups enforce the requested exclusivity and are covered by tests.

Written by the indexing model from the issue text.