Include the version of your package in our source code.

1. Add build_version to pubspec.yaml. Also make sure there is a version field.

   name: my_pkg
   version: 1.2.3
   dev_dependencies:
     build_runner: ^1.0.0
     build_version: ^2.0.0

2. Run a build.

   > pub run build_runner build

   lib/src/version.dart will be generated with content:

   // Generated code. Do not modify.
   const packageVersion = '1.2.3';