describe-package: fefchFileListing throws a 404 for @latest version

Can't believe I didn't notice this before,  but I can't see an issue

If you call `fetchFilelisting` with an `@latest` version, you'll get a 404 back and an error will be thrown.

The function should be smart enough to resolve `@latest` to a version number.

Maybe we need https://www.jsdelivr.com/docs/data.jsdelivr.com#get-/v1/packages/npm/-package-/resolved