Collapsed imports; rewrite source to not break compilation

https://ellie-app.com/3Wd7CZC2Mkya1
```
module Main exposing (main)

import Browser
import Browser as B

x = 3
```
elm-formatted into
```
module Main exposing (main)

import Browser as B

x =
3
```
and no references to `Browser.thing` were changed into `B.thing`.

Can we translate all `Browser.thing` into `B.thing` if we collapse import statements so it doesn't break code?

Related: #379 #445

Use the linked Ellie reproduction as the first check, then inspect the formatter logic for collapsing imports and handling qualified references. Done means the example remains compilable after imports collapse, with references updated consistently; review the related cases in #379 and #445.

Written by the indexing model from the issue text.