* builtin/providers: implement terraform remote state datasource as providers.Interface
* append and return diags separately (to match the idiomatic usage
elsewhere in Terraform)
* diagnostic summary style improvements
* update tests to pass config to schema.CoerceValue
* trust that the schema will be enforced and there is no need to check
that a given attribute exists
* added dataSourceRemoteStateGetSchema() (effectively replacing a
function that was inappropriately removed) for consistency with other
terraform providers
* builtin/provider terraform test: added InternalValidate() test for dataSourceRemoteStateGetSchema
The "terraform" provider was previously split out into its own repository,
but that turned out to be a mistake due to how tightly it depends on
aspects of Terraform Core.
Here we prepare to bring it back into the core repository by reorganizing
the directory layout to conform with what's expected there.
