[native_assets_builder] Move packageLayout to builder constructor

[dcharkes]

All Dart and Flutter commands have a single package config and runPackageName.

Part of a larger refactoring that will make the NativeAssetsBuildRunner take more arguments in the constructor such as the package config, and reduces the number of arguments to the build and link methods.

This will enable:

• Caching BuildPlanner logic (only one runPackageName)
• Changing packagesWithAssets to take runPackageName into account (and possibly moving it to the native assets builder instead). [native_assets_builder] Support pub workspaces #1905

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

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: 88.557%. first build
when pulling 4e6f73a on move-package-layout
into b2ad4a8 on main.