External commands: First argument should be the name the command was invoked at

[mpickering]

The rust external command system passes the name of the command which was invoked to the executable as the first argument.

This is worthwhile because then you can implement multiple external commands using the same executable but provide different symlinks to provide different commands to cabal.

I don't see why we shouldn't implement this same interface.

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