[native_assets_cli] Simplify library structure

[dcharkes]

Closes: #1805

This PR merges the _testing and _builder libraries into the "hook" libraries.

This leads to one library per package once we split it up in package:hooks, package:data_assets, and package:code_assets (#999).

Upside:

• Only one import instead of two for the following use cases:
  • tests testing hooks (constructing Inputs and reading Outputs)
  • hook helper packages (constructing Inputs and reading Outputs in its tests)
• This leads to more classes available in scope than needed in the following use case:
  • hooks (only reading Inputs, and writing Outputs)

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
native_assets_builder	Non-Breaking	0.17.0	0.18.0-wip	0.17.1	✔️
native_assets_cli	Breaking	0.17.0	0.18.0-wip	0.18.0	✔️
native_toolchain_c	Non-Breaking	0.14.0	0.15.0-wip	0.14.1	✔️

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 symbols

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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

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

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
native_assets_builder	Non-Breaking	0.17.0	0.18.0-wip	0.17.1	✔️
native_assets_cli	Breaking	0.17.0	0.18.0-wip	0.18.0	✔️
native_toolchain_c	Non-Breaking	0.14.0	0.15.0-wip	0.14.1	✔️

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 symbols

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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

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

[coveralls]

coverage: 77.937% (-0.5%) from 78.419%
when pulling 397768c on cleanup-imports-5
into deda4d1 on main.

[goderbauer]

LGTM