Point at path segment on module not found

Point at the correct path segment on a import statement where a module
doesn't exist.

New output:

```rust
error[E0432]: unresolved import `std::bar`
 --> <anon>:1:10
  |
1 | use std::bar::{foo1, foo2};
  |          ^^^ Could not find `bar` in `std`
```

instead of:

```rust
error[E0432]: unresolved import `std::bar::foo1`
 --> <anon>:1:16
  |
1 | use std::bar::{foo1, foo2};
  |                ^^^^ Could not find `bar` in `std`

error[E0432]: unresolved import `std::bar::foo2`
 --> <anon>:1:22
  |
1 | use std::bar::{foo1, foo2};
  |                      ^^^^ Could not find `bar` in `std`
```

src/test/ui/span/non-existing-module-import.rs

@@ -0,0 +1,13 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::bar::{foo1, foo2};
+
+fn main() {}

src/test/ui/span/non-existing-module-import.stderr

@@ -0,0 +1,8 @@
+error[E0432]: unresolved import `std::bar`
+  --> $DIR/non-existing-module-import.rs:11:10
+   |
+11 | use std::bar::{foo1, foo2};
+   |          ^^^ Could not find `bar` in `std`
+
+error: aborting due to previous error
+