These are similar to the symbols of the same name in package "providers".
terraform.ProvisionerFactory is now an alias for provisioners.Factory, so
we can defer updating all of the existing users of it.
Close was previously an optional interface, because it's only applicable
on the client side but the types were shared accross all packages. Since
all plugins will now implement Close, it no longer needs to be optional.
The "config" package is no longer used and will be removed as part
of the 0.12 release cleanup. Since configschema is part of the
"new world" of configuration modelling, it makes more sense for
it to live as a subdirectory of the newer "configs" package.
