fix(derive): Use fully qualified types

[janikrabe]

This fixes #[derive(Parser)] when a custom Result type is in scope.

For example, this code fails to compile without this patch:

use clap::Parser;

type Result<T> = result::Result<T, ()>;

#[derive(Parser)]
pub struct Options {}

[janikrabe]

@sanpii was faster #3091. My PR also updates an Option in dummies.rs. I believe this code is only emitted when compilation has already failed but it might be a good idea to qualify the type anyway to make sure the error message isn't more confusing than it needs to be.

[epage]

Thanks!

[pksunkara]

@epage We might want to mention about this derive's contributing doc.

[epage]

An rc.1 is now released

(and contributing doc was updated)