two runs required on imports with leading ::

Formatting can take two runs when imports are using absolute paths and include leading :: (when using the 2018 edition config option)

Original source:

use ::foo;
use ::foo::{Bar};
use ::foo::{Bar, Baz};
use ::{Foo};
use ::{Bar, Baz};

1st formatting result:

use ::foo;
use ::foo::Bar;
use ::foo::{Bar, Baz};
use ::Foo;
use ::{Bar, Baz};

And then second formatting run to get the final result:

use ::Foo;
use ::foo;
use ::foo::Bar;
use ::foo::{Bar, Baz};
use ::{Bar, Baz};

Seems to be related to the first run reducing ::{Foo} to ::Foo but without fully sorting/reordering the set of imports, and then the second run sorting them to finish formatting.

Reproduce the issue with the Rust import examples in the report, using the 2018 edition configuration. Trace the import formatting and sorting entry points involved in reducing leading-:: imports; done means one formatting run produces the final ordering and a second run makes no further changes.