These are classes that use NO core classes of Vagrant, and are therefore
safe to load for upgrades. i.e. a V2 core can load a V1 config class
that is deemed upgrade safe without crashing Vagrant.
Before we were manually going over every plugin and getting each piece,
all over the place. Now we have a central manager that will give us all
the pieces we want. There is still some cleanup to do here but this is
much better overall.
