When working on #2803, I noticed a disprecancy in the augment
behavior between `Arg`s and `Subcommand`s that makes it so `Arg`s can't
be flattened with the update functionality.
Before #2005, `Clap` was a special trait that derived all clap traits it
detected were relevant (including an enum getting both `ArgEnum`,
`Clap`, and `Subcommand`). Now, we have elevated `Clap`, `Args`,
`Subcommand`, and `ArgEnum` to be user facing but the name `Clap` isn't
very descriptive.
This also helps further clarify the relationships so a crate providing
an item to be `#[clap(flatten)]` or `#[clap(subcommand)]` is more likely
to choose the needed trait to derive.
Also, my proposed fix fo #2785 includes making `App` attributes almost
exclusively for `Clap`. Clarifying the names/roles will help
communicate this.
For prior discussion, see #2583
When using `#[clap(flatten)]` inside of a `Subcommand`, we would do a
`from` instead of an `update`.
The challenge is knowing when we are
going into a flattened subcommand vs changing the variant. To resolve
this, I added a `Subcommand:has_subcommand(name)` trait method that we
generate, so we can ask.
