that is referenced by multiple `extern crate` items.

For the following code:

```rustc
struct Bar;
struct Bar;

fn main () {
}
```

show

```nocode
error[E0428]: a type named `Bar` has already been defined in this module
  --> src/test/compile-fail/E0428.rs:12:1
   |
11 | struct Bar;
   | ----------- previous definition of `Bar` here
12 | struct Bar;
   | ^^^^^^^^^^^

error: aborting due to previous error
```

instead of

```nocode
error[E0428]: a type named `Bar` has already been defined in this module
  --> src/test/compile-fail/E0428.rs:12:1
   |
11 | struct Bar;
   | ----------- previous definition of `Bar` here
12 | struct Bar;
   | ^^^^^^^^^^^

error[E0428]: a value named `Bar` has already been defined in this module
  --> src/test/compile-fail/E0428.rs:12:1
   |
11 | struct Bar;
   | ----------- previous definition of `Bar` here
12 | struct Bar;
   | ^^^^^^^^^^^

error: aborting due to 2 previous errors
```

Given a file

```rust
use std::collections::{BinaryHeap, BTreeMap, BTreeSet};

fn main() {}
```

Show a single warning, instead of three for each unused import:

```nocode
warning: unused imports, #[warn(unused_imports)] on by default
 --> foo.rs:1:24
  |
1 | use std::collections::{BinaryHeap, BTreeMap, BTreeSet};
  |                        ^^^^^^^^^^  ^^^^^^^^  ^^^^^^^^
```

Include support for lints pointing at `MultilineSpan`s, instead of just
`Span`s.

Given the following code:

```rust
struct Foo<T: Clone>(T);

use std::ops::Add;

impl<T: Clone, Add> Add for Foo<T> {
    type Output = usize;

    fn add(self, rhs: Self) -> Self::Output {
      unimplemented!();
    }
}
```

present the following output:

```nocode
error[E0404]: `Add` is not a trait
 --> file3.rs:5:21
  |
5 | impl<T: Clone, Add> Add for Okok<T> {
  |                ---  ^^^ expected trait, found type parameter
  |                |
  |                type parameter defined here
```

This speeds up compilation by 3--6% across most of rustc-benchmarks.

Diagnostics for struct path resolution errors in resolve and typeck are unified.
Self type is treated as a type alias in few places (not reachable yet).
Unsafe cell is seen in constants even through type aliases.
All checks for struct paths in typeck work on type level.

I had it a couple of times that I was missing the "extern crate" line
after I introduced a new dependency. So I copied the text from the
message and inserted it into the beginning of my code, only to find the
compiler complaining that I was missing the semicolon. (I forgot to add
it after the text that I had pasted.)

There's a similar message which does include the semicolon, namely
"help: you can import it into scope: `use foo::Bar;`". I think the two
messages should be consistent, so this change adds it for "extern
crate".