Rename prefix needs better handling on shadowing names

[FMorschel]

Based on both:

• [analyzer] Confusing behaviour when renaming constructor declarations #61008
• Add a code action on imports to add/remove aliases Dart-Code/Dart-Code#4289

I'd started working on https://dart-review.googlesource.com/c/sdk/+/421543. I'll leave that one for creating the new actions, but it was stale because while developing it I found out that the rename prefix really needs some attention on shadowing, it has basically nothing today (both for removing the name entirely and adding a new name that shadows another declaration).

So this issue is intended to track only the handling of shadowing names when renaming the prefix. I'll remove the work on this that I had started on that CL (last patchset to have this was 4), and later I might come back to this, but if anyone knows better how to handle this and wants to jump on it before I do, please feel completely free to do so.

Here are some examples of tests I had added:

import 'dart:core'; //Try adding `core`

void f() {
  var core = 42;
  print(core);
}

import 'dart:core' as c^ore;

// Maybe try importing and using too
void print(String s) {}

void f() {
  print('Hello, world!');
  core.print('Hello, world!');
}

FYI @bwilkerson

[bwilkerson]

I agree that the rename refactoring needs to take shadowing into account.

Whatever code we write to check whether it's safe to change the name should be written to be reusable for the 'add' and 'remove' actions that we want to add.