Generate code to a specific directory

[sadar]

In the build.yaml by specifying the build_extensions property i was not able to generate the code to a specific directory. Every time the generated code is in the same directory as of the source file. Is there a way to fix this?

[schultek]

This would need to be implemented for this package, its not something thats built into build_runner.
I'm happy to review a PR, but I'm short on time right now to do it myself.