[feature request] Command `--help` should show available subcommands

Nobody has claimed this yet.

```js
prog
.command('foo')
.describe('Top level command')
.action(() => console.log('see `app foo --help`'))

prog
.command('foo bar')
.describe('A subcommand')
.action(() => console.log('did a thing'))
```

Use case: the `foo` command doesn't do anything and requires usage of subcommands. As it is, doing `app foo --help` does not display any possible subcommands under the "Usage" section. Instead, one has to do `app --help` to see a potentially long list of available commands and subcommands. Ideally, `app --help` would show the `foo` command but not the `foo bar` command so that one would have to do `app foo --help` to see the available subcommands.

Side request: `foo` _could_ do something, but it'd just be an alias of a required subcommand.

Start by tracing how command registration and help output handle the examples `foo` and `foo bar`. Check the output of `app --help` and `app foo --help`; done means the top-level help lists `foo` without `foo bar`, while the nested help lists the available subcommand.

Written by the indexing model from the issue text.