As part of the 0.10 core/provider split we moved this provider, along with
all the others, out into its own repository.
In retrospect, the "terraform" provider doesn't really make sense to be
separated since it's just a thin wrapper around some core code anyway,
and so re-integrating it into core avoids the confusion that results when
Terraform Core and the terraform provider have inconsistent versions of
the backend code and dependencies.
There is no good reason to use a different version of the backend code
in the provider than in core, so this new "internal provider" mechanism
is stricter than the old one: it's not possible to use an external build
of this provider at all, and version constraints for it are rejected as
a result.
This provider is also run in-process rather than in a child process, since
again it's just a very thin wrapper around code that's already running
in Terraform core anyway, and so the process barrier between the two does
not create enough advantage to warrant the additional complexity.
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