What would be the recommended way to copy a source code file from a KSP project into the generated sources folder of a project consuming my plugin?

Make it a normal dependency?

When your gradle plugin registers, you can inject your ksp module as a dependency to relevant sourceSet. Both needs to be published before this works. However this is a bit intrusive and not a standard practice. Usually you expect a user to applu your plugin, apply ksp plugin and add your processor as ksp dependency to relevant sourceSet.

If the KSP project is the project where you define the generator, you could always have your generator explicitly generate that file.