This is a rather-messy, complex change to get the "command" package
building again against the new backend API that was updated for
the new configuration loader.
A lot of this is mechanical rewriting to the new API, but
meta_config.go and meta_backend.go in particular saw some major
changes to interface with the new loader APIs and to deal with
the change in order of steps in the backend API.
The usual way to use a configschema.Block is to obtain a hcldec spec from
it and then decode an hcl.Body. There are inevitably situations though
where a body has already been decoded into a cty.Value before we know
which schema we need to use.
This new method CoerceValue is intended to deal with this case, applying
the schema to an already-decoded value in what should be an intuitive way
for most situations.
We will need access to this information in order to render interactive
input prompts, and it will also be useful in returning schema information
to external tools such as text editors that have autocomplete-like
functionality.
We don't currently have any need for this information, but we're
propagating it out of helper/schema here pre-emptively so that once we
later have a use for it we will not need to rebuild the providers to gain
access to it.
The long-term expected use-case for this is to have Terraform Core use
static analysis techniques to trace the path of sensitive data through
interpolations so that intermediate results can be flagged as sensitive
too, but we have a lot more work to do before such a thing would actually
be possible.
zcldec now has its own function for computing the implied type for a spec,
so we can use that instead of our own logic.
The zcldec logic is more general since its spec model is more general than
our schema model here, but it produces the same results for the subset
of specifications that our DecoderSpec method produces.
This returns a cty.Type that the caller can expect to recieve when
decoding a value using the (not yet implemented) decoder specification
for a given schema.
Martin Atkins