A common new-user mistake is to place variable _declarations_ into .tfvars
files instead of variable _values_. To guide towards the correct approach
here, we add a specialized error message for that situation that includes
guidance on the distinction between declaring and setting values for
variables, and an example of what setting a value should look like.
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.
Martin Atkins