Clearer Error Message for Duplicate Definition

Clearer use of the error message and span labels to communicate
duplicaiton defitions/imports.

New error format:

```
error[E0428]: the name `Foo` is defined twice
 --> example.rs:2:1
  |
1 | trait Foo { }
  | ------------- previous definition of the trait `Foo` here
2 | struct Foo { }
  | ^^^^^^^^^^^^^^ `Foo` redefined here
  = note: `Foo` must be defined only once in the type namespace of this module

error: aborting due to previous error
```

0 files changed, 0 insertions, 0 deletions