Resolve needs a better error message for some imports

[emberian]

All imports are relative to the root of the crate, not the current mod. This is counterintuitive and the error messages are rather unhelpful. For example, a use foo; should, when resolve fails, look in self to see if there is a foo and in the error message suggest using use self::foo.

Part of #7379.

[emberian]

This interacts especially poorly with extern mod's, from #7583:

mod a {
    extern mod extra;
    use extra::json;
}

fn main() {}

generates

/tmp/b.rc:4:8: 4:13 error: unresolved import. maybe a missing `extern mod extra`?
/tmp/b.rc:4     use extra::json;
                    ^~~~~
/tmp/b.rc:4:8: 4:19 error: failed to resolve import `extra::json`
/tmp/b.rc:4     use extra::json;
                    ^~~~~~~~~~~
error: aborting due to 2 previous errors

[catamorphism]

Triage bump. Agreed it should be better, but not a release blocker.

[mrshu]

I would like to try to tackle this one by at least providing a better explanation of what is going on.

I was able to figure out that it will be necessary to alter librustc/middle/resolve.rs but I am not sure how to look in self as @cmr pointed out. Could please someone help me out with this? Thanks.

[treeman]

Same as #8640.

[alexcrichton]

This I believe was actually closed in 207bfee