(PUP-3190) Don't assume puppet is in a lib dir

The loaders previously assumed that any installation of puppet would be
under a directory called lib. This holds true when running puppet from
source (during development), or in some package cases, but is not
generally true. This simplifying assumption was made so that the loading
path for both puppet system extensions (built in functions) and module
extensions could be loaded in the same manner: find the root of the
extender and loading code from lib/puppet/....

This changes to loading to instead only assume that there is a puppet
directory and not the name of the parent. This, however, forces the
loader for modules to become specialized to only be able to find ruby
extentions, because they are in the 'lib' directory, which is how the
loader must now be configured.