This improves error reporting for the following class of imports:
```rust
use foo::bar;
```
Where foo, the topmost module, is unresolved. It now results in:
```text
/tmp/foo.rs:1:4: 1:7 error: unresolved import. perhapsyou forgot an 'extern mod foo'?
/tmp/foo.rs:1 use foo::bar;
^~~
/tmp/foo.rs:1:4: 1:12 error: failed to resolve import: foo::bar
/tmp/foo.rs:1 use foo::bar;
^~~~~~~~
error: failed to resolve imports
error: aborting due to 3 previous errors
```
This is the first of a series of changes I plan on making to unresolved name error messages.
f04eb37c7e