* Split node_resource_abstract.go into two files, putting
NodeAbstractResourceInstance methods in their own file - it was getting
large enough to be tricky for (my) human eyeballs.
* un-exported the functions that were created as part of the EvalTree()
refactor; they did not need to be public.
When we need to select a qualified provider address based on an implied
provider name, we have a special case that the name "terraform" maps to
terraform.io/builtin/terraform instead of
registry.terraform.io/hashicorp/terraform as would be the case for other
prefixes.
However, in order for that to work properly we need to use
addrs.ImpliedProviderForUnqualifiedType instead of
addrs.NewDefaultProvider, because the latter just unconditionally always
produces a "default" provider configuration (belonging to the "hashicorp"
namespace on the public registry).
