Because we gradually build the `ArgGroup` as we parse the YAML, we don't
use `ArgGroup::new`. Clap3 introduced an internal `id` in addition to
the public `name` and it appears that this custom initialization code
was not updated.
This shows the problem with publically exposing `impl Default`.
Choices:
- Remove `impl Default`
- Always valid
- Requires spreading invariants
- Callers can't implement code the same way we do
- Add `ArgGroup::name`
- Can be constructed in an invalid state
- Centralizes invariants
- A caller could implement code like the yaml logic
I decided to go with `ArgGroup::name`.
Fixes#2719
