Include the package name as a constant

Hello,
Would it be possible to include the package name as a Dart constant?

For a project which uses Protobuf there is a versioning system in the package name in order to establish a compatible communication between client and server. Example: `package Project.v1_3`. Currently I need to update a constant manually every time the protocol versioning changes, where I could simply use an autogenerated constant from the proto file.

Start at the Dart code-generation entry point that reads a .proto file and trace how its package name is handled. Define the expected generated constant and verify that a package such as Project.v1_3 produces the correct value, including a test for the generated output.

Written by the indexing model from the issue text.