I got an email from http://itch.io\|itch.io asking me to upload files with my submission. Since I’m submitting a tool implemented as a Racket package, what format should my upload be? Output from raco distribute?

Is it a separate runnable tool or a library?

If the former then yeah raco distribute is fine

It can be used as either. There’s a launcher and usable modules. I’m guessing a GitHub link on the http://itch.io\|itch.io page is enough to showcase it as a library?

Yes