rustc: Give a hint for missing self/super for imports & reduce number of redundant errors

It could be, but the reason for the changes was it was inconsistent with other resolve errors, e.g. "unresolved name blah. Did you mean self.blah". When I sampled the errors it seemed better to have consistency one way or the other. 

If I change to `: {}`, should I look to port the other `Did you mean` errors to the same?