Rust 2018: cargo fix doesn't remove extern crate

[Procrat]

From what I read in the edition guide, extern crate is now redundant. cargo fix didn't remove these statements however, causing the following two errors after switching to Rust 2018 (with uniform paths):

error: `chrono` import is ambiguous
  --> src/...
   |
5  | extern crate chrono;
   | -------------------- can refer to `self::chrono`
...
47 | use chrono::prelude::*;
   |     ^^^^^^ can refer to external crate `::chrono`
   |
   = help: write `::chrono` or `self::chrono` explicitly instead
   = note: relative `use` paths enabled by `#![feature(uniform_paths)]`

The second error I got feels like it should have been a warning:

error: `take_mut` import is redundant
 --> src/...
  |
8 | use take_mut;
  |     ^^^^^^^^
  |     |
  |     refers to external crate `::take_mut`
  |     defines `self::take_mut`, shadowing itself
  |
  = help: remove or write `::take_mut` explicitly instead
  = note: relative `use` paths enabled by `#![feature(uniform_paths)]`

cargo fix should probably also remove these redundant use statements?

Given some pointers, I'm happy to take a look at both/either this issue and/or the other one I filed!

[zackmdavis]

Duplicate of #53797: I just left a comment with implementation hints there.

[alexcrichton]

Indeed!