Offer suggestions for missing libraries

[brson]

In #5729 we tried to make rustc offer suggestions when it thinks you forgot to write extern mod foo;. That pull request didn't land but this seems like a reasonable idea.

[alexcrichton]

This seems to currently be implemented:

use bar::baz;

$ rustc foo.rs
foo.rs:1:5: 1:8 error: unresolved import. maybe a missing `extern crate bar`?
foo.rs:1 use bar::baz;
             ^~~
foo.rs:1:5: 1:13 error: failed to resolve import `bar::baz`
foo.rs:1 use bar::baz;
             ^~~~~~~~
error: aborting due to 2 previous errors

Closing (as this seems to be done)