(#6962) Integrate legacy subcommands into the help face.

Previously we would only emit help for a face; now we fully integrate legacy
subcommands in the sense that asking for face-level help will emit their
entire help text.

Asking for any action subsequent will raise an error: this is an annoyingly
inconsistent behaviour, but there isn't a saner definition of the change.

Reviewed-By: Matt Robinson <matt@puppetlabs.com>