Tracking improvements to group_imports

[xStrom]

We would like to use the rustfmt setting group_imports = "StdExternalCrate".

The problem with it is that it mixes different visibility cases. As mentioned in the tracking thread:

At the moment, it produces the following kinds of use lists:

use a;
pub(crate) use b;
pub use c;
use d;
pub use x;
pub(crate) use z;

whereas it feels more natural for the sort order to be:

pub use c;
pub use x;
pub(crate) use b;
pub(crate) use z;
use a;
use d;

That is, sort by visibility then by item name.

[DJMcNab]

I'd probably also want those groups to be seperated with blank lines (and I'd probably advocate for the order to be the other way around (so that the public API doesn't have imports in the middle of it).

But agreed, we should work with upstream to improve this.