[Feature Request] Allow Deep Links validation tool to support flavors (not only `lib/main.dart` entry point)

The [tool](https://docs.flutter.dev/tools/devtools/deep-links) for validating deep links does not allow to specify an entry point for the app other than the default one (`lib/main.dart`). If I have flavors with multiple main files (such as `main_development.dart`, `main_staging.dart`, etc), which is a common case in many applications, particularly enterprise ones, I'm not able to use the tool.

Start with the deep-links validation tool described in the linked documentation and trace how it currently selects lib/main.dart. Identify the relevant implementation and tests, then make completion mean that a flavor-specific entry point such as main_development.dart can be selected and validated.

索引モデルが issue の本文から書いたものです。