Before, when doing an `update` involving subcommands, we generated
parsing rules for the `from` case instead, requiring all arguments to be
present.
This switches us to descending into `update` code and adds tests to
verify it works.
This is a part of #2605
`structopt` originally allowed
```
pub enum Opt {
Daemon(DaemonCommand),
}
pub enum DaemonCommand {
Start,
Stop,
}
```
This was partially broken in #1681 where `$ cmd daemon start` works but `cmd daemon`,
panics. Originally, `structopt` relied on exposing the implementation
details of a derived type by providing a `is_subcommand` option, so we'd
know whether to provide `SubcommandRequiredElseHelp` or not. This was
removed in #1681Fixes#2005
