[native_assets_builder] Rename `build.dart` to `hook/build.dart`

[dcharkes]

We have settled on hook/build.dart as the place for the build.dart "script"/"hook".

• 🪝 [breaking change] Reserve top-level hook/ directory 🪝  sdk#54334

If we fall back to build.dart for the time being, we have less rolling issues.

The example that's run against the Dart SDK will break for the time being though.

• Update native_assets_builder to look for hook/build.dart instead of build.dart. For now: fall-back to build.dart if hook/build.dart does not exist, to not immediately break everyone.
• Update tests.
• Update "script" terminology to "hook".
• Roll into Dart.
• Update examples (requires Dart roll).
• Roll into Flutter. (currently blocked by [native_assets_cli] DynamicLoadingBundled and dryRun in Flutter #1049)

[dcharkes]

It will be easier to roll stuff if we first land build_assets.dart to preceed build.dart roll everything, and then remove build.dart.

[dcharkes]

Note from offline discussion: We might want to stick with build.dart. So far we have only a single conflicting use: flutter/flutter#138727 (comment).

[mit-mit]

I still prefer the more general names. We may only know of a single conflict now, but having a hard coded name is a very brute mechanism, so I think it should be more specific.

[dcharkes]

(moved list to top)