This new source type should be used for variables loaded from .tfvars files that were explicitly passed as command line arguments (e.g. -var-file=foo.tfvars)
In the new implementation of collecting variables I initially forgot the
JSON variant of terraform.tfvars.
This fix is verified by TestApply_varFileDefaultJSON now passing.
This connects a missing link left by earlier refactoring: the command
package is responsible for gathering up variable values provided by the
user and passing them through to the backend to use in operations.
