The code in lib/src in your Dart project isn't publicly exposed to other modules in your project. It is considered as private.
This is one of the structural advantages of the packaging system in Dart.
In case you want to export code present in the files in lib/src, you can use the export keyword in Dart.
Here's an example:
export 'src/file1.dart';
export 'src/file2.dart';
export 'src/handlers/file3.dart';
export 'src/viewmodels/file4.dart';