Refactor environment setup.

- Gave setup_environment and setup_dependent_environment more similar
  signatures. They now allows editing the Spack env and the runtime
  env for *this* package and dependents, respectively.

- modify_module renamed to setup_dependent_python_module for symmetry
  with setup_dependent_environment and to avoid confusion with
  environment modules.

- removed need for patching Package objects at runtime.

- adjust packages to reflect these changes.