External commands: `cabal help <cmd>` does not work

[mpickering]

cargo interprets cabal help <cmd> as cabal-cmd --help, this makes external commands integrate nicely into the existing help interface.

(cf https://doc.rust-lang.org/cargo/reference/external-tools.html#custom-subcommands)

[Mikolaj]

@yvan-sraka: any comments? Feel free to remove the "needs triage" label once you think this does not need any further discussion.