Forbid args to conflict with themselves

[matthiaskrgr]

I just found something like this in my code
Arg::with_name("a").conflicts_with("a")
This makes it impossible to use a because it will always cause an error.
I wonder if we could have some kind of warning put out if an arg conflicts with itself (preferably at buildtime, but I'm not sure if this can be done yet).

clap 2.33.0

[CreepySkeleton]

It's impossible to issue warnings depending on certain conditions, but we can panic in this case.