(PUP-1640) Add test of env and module data-composition using function

This adds a test that:

* asserts that data from a module has lower precedence that data from
  an environment
* the function data mechanism works for both env and module
* the function data mechanism calls the expected functions
* the configuration of extensions allowing the specification of
  data mechanism works for env and module.
(PUP-1640) Add test of module/env data provider via function

This also removes unused .pp files.