☂️ [native_assets_cli] Overhaul dart API for `build.dart`

[dcharkes]

package:native_assets_cli should only have an API for implementors of build.dart (and later link.dart). Any other implementation details should be hidden.

Moreover, the API that is in package:native_assets_cli should be well documented.

• Move all code that is only used by Dart/Flutter SDK to package:native_assets_builder
• [native_assets_cli] BuildOutput API #25
• [native_assets_cli][native_assets_builder] CLI input and output composability #853
• [doc] Document how to do build.dart that uses packages from a local native package manager #189
• [doc] Document how to do build.dart that downloads a binary #157
• [native_assets_cli] Dart API interface per asset type #994
• [native_assets_cli] Explore using extension types  #997
• [native_assets_cli] Package name #999