Rolls the packages from https://github.com/dart-lang/native in the native assets implementation.

Most notable we're refactoring `package:native_assets_cli` for `build.dart` use.
Therefore, all imports to that package for Flutter/Dart should be to the implementation internals that are no longer visible for `build.dart` writers. Hence all the import updates.

No behavior in Flutter apps should change.

This PR also updates the template to use the latests version of `package:native_assets_cli` which no longer exposes all the implementation details.