(PUP-2035) Fix issues with loader configuration

During the review of the functionality for PUP-2035 it was discovered
that it was not possible to call a function in a module from the
environment. This was due to the initial configuration, and that the
evaluator used the wrong loader.

This commit fixes the logic so that:

• the environment loader is now split into two parts, a public (sees

only things in the environment), and private (the public + all modules).

• a module loader is parented by the public_environment_loader (does not

see all modules - only those it depends on).

• functions defined outside of a module are added to the public loader

(i.e. from the command line, or site.pp)

This fix is needed to be able to evaluate PUP-2176