fix: make imports_granularity One preserve aliases

[willbuckner]

Fix imports_granularity = "One" to preserve aliases.

In the following example by sivizius:

pub use foo::x;
pub use foo::x as x2;
pub use foo::y;
use bar::a;
use bar::b;
use bar::b::f;
use bar::b::f as f2;
use bar::b::g;
use bar::c;
use bar::d::e;
use bar::d::e as e2;
use qux::h;
use qux::i;
use qux::i as j;

bar::b::f as f2; and qux::i as j; were silently dropped, returning this merged result:

  pub use foo::{x, x as x2, y};
  use {
      bar::{
          a,
          b::{self, f, g},
          c,
          d::{e, e as e2},
      },
      qux::{h, i},
  };

Two import paths that only differ by the alias of their last segment were being treated as equal when merging, keeping only one of the two names. They now get merged into a list containing both, e.g., qux::{h, i, i as j}.

This also fixes two related problems:

• Merging use qux::h; followed by use qux as Q; produced the invalid use qux as Q::{self as Q, h}; because the merged root kept the alias of the shorter path.
• The result of merging depends on the order in which the use trees get visited, and once aliases are preserved a single pass over inputs like use a; use a as b; use a::c; did not, produce a stable result. a::{self as b} is now normalized to the equivalent a as b when flattening, and merging is repeated until it reaches a fixed point, so that formatting stays idempotent.

Fixes: #6027