In Proejct 1, structopt is an extension part. But with clap 3.0, structopt is already built-in. See https://github.com/clap-rs/clap#how-does-clap-compare-to-structopt

clap is structopt. With the 3.0 release, clap imported the structopt code into it's own codebase as the clap_derive crate.

I think we can mention that this extension is only needed if clap < 3.0 is used?