[ffigen] Fix decl_decl_collision_test.dart

[liamappelbe]

There were 2 bugs here.

• One issue was that since the transformer refactor, this test's bindings weren't being transformed, so none of the renaming was being applied. Just had to update the test to fix this.
• The other issue is that the renamings weren't taking into account the imported libraries (eg import 'dart:ffi' as ffi; would conflict with bindings named ffi). There was an existing step that renamed user imported libraries, so I've just extended it to also rename built-in imports like ffi.

Fixes #1889

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	4.1.0	5.0.0-wip	5.0.0	✔️

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

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License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 91.459%. first build
when pulling ab72f83 on fix1889
into 817c76e on main.