[ffigen] Prevent imported types from being renamed.

[liamappelbe]

Fix #2762

Adding a flag that just prevents these symbols from being renamed feels like a bit of a hack. So I also tried another approach, but that turned out to be a lot more brittle than this approach.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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pkgs/objective_c/lib/src/ns_input_stream.dart

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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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[coveralls]

coverage: 87.585% (+0.1%) from 87.445%
when pulling 208768a on ffigen_sym_import
into 5501355 on main.