[racket] errors from raco setup while installing github Racket

At Wed, 2 Jul 2014 23:56:32 -0700, Matthew Butterick wrote:
> Update 2: my own fault. Somehow I accidentally did `raco pkg install` while 
> standing on the git/racket directory. That was a little too recursive, 
> apparently. Now sorted.

I've changed `raco pkg install` so that it rejects package links that
would overlap with existing package or collection paths.