Add module name constant to class for downstream use (#38295)

Summary:
Pull Request resolved: #38295

Move constant to class instance so customers can use strongly typed name, feedback from D46159001

Changelog:
[Internal] [Changed] - Add module name constant to codegen'd class for downstream use

Reviewed By: christophpurrer

Differential Revision: D47095993

fbshipit-source-id: 741d0d837bf912d6b32e5f12c5df871563d46686

Author: rbergerjr