[ffigen] Remove headers from BindingIndex

[liamappelbe]

Working on simplifying the BindingIndex. First step is to remove the _headerCache, since it doesn't really fit. It's the only part of the index that's not keyed by USR.

[coveralls]

coverage: 82.207% (-0.9%) from 83.08%
when pulling e81ed96 on ffigen_cleanup_4
into 77c2e52 on main.

[github-actions]

PR Health

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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no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/hooks_runner/test_data/download_assets/hook/build.dart
pkgs/objective_c/example/command_line/lib/main.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

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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.

Package	Leaked API symbol	Leaking sources

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Breaking changes ✔️

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