PUP-3336 split $libdir when updating $LOAD_PATH

As reported, $libdir can (but typically won't) take a multi-part
libdir.  This was being used wholesale in updating $LOAD_PATH, but
ruby have any special treatment, so would just have an invalid path in
$LOAD_PATH.

With this commit we split libdir when updating $LOAD_PATH, to ensure
we're updating each component correctly.
Yeah, well we could solve this by issuing a bold declaration that it's always just a single path, remove the split from autoload.rb and supply patches to rspec-puppet.

https://github.com/rodjek/rspec-puppet/blob/master/lib/rspec-puppet/support.rb#L140

Let's aim for consistently consistent, at least here.