Remove `extern crate` when migrating to Rust 2018

[johnthagen]

Migrated from rust-lang/cargo#5829

It's been mentioned/requested multiple times on the internals thread that cargo fix should remove the extern crate statements when moving to Rust 2018.

Motivation

One of the big draws of Rust 2018's module system is that all of the "redundant" extern crate lines can finally be removed. It took some of the excitement away when I went through all of the cargo fix steps and still saw them there. I think it will just be a better experience for users to see one of the tangible benefits of this change. As it is right now, they just see the crate:: in front of all of their local imports and they might not realize what a nice change the module system updates really are.

After I manually removed the extern crate's I did get my satisfying result 😄, but let's give this to users on the first go around.

Implementation

If I understand correctly this should be implemented in the compiler under the #![warn(rust_2018_idioms)] lints, rather than cargo fix itself.

cc @alexcrichton

[alexcrichton]

I'm moving this forward up on the milestone list due to just having closed #52451 in favor of this issue, which was itself on the RC milestone.

[alexcrichton]

Ok so I've some triaging of extern crate related lint issues, and I believe the current work items are:

• A rustfix-fixable suggestion for removing extern crate needs to be implemented. The implementation would go here.
• Currently the lint suggests switching to simply use. While technically correct and handles lots of corner cases, this doesn't achieve the desired goal of actually removing annotations.
• We can't blanket remove all extern crate statements, for example this test case is correctly warned to use a use rather than deleting the statement outright.

I'm testing out some ideas locally as well.

[alexcrichton]

I believe this should be fixed by #53684, I think the naive solution actually works well!