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

Summary:
Pull Request resolved: facebook#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

Differential Revision: https://www.internalfb.com/diff/D47095993?entry_point=27

fbshipit-source-id: dfa0655dee5428e81063b9ef755d0fac7d3735e3

Author: rbergerjr