Now that variable evaluation checks for a nil expression the graph
transformer does not need to generate a synthetic expression for
variable defaults. This means that all default handling is now located
in one place, and we are not surprised by a configuration expression
showing up which doesn't actually exist in the configuration.
Rename nodeModuleVariable.evalModuleCallArgument to evalModuleVariable.
This method is no longer handling only the module call argument, it is
also dealing with the variable declaration defaults and validation
statements.
Add an additional tests for validation with a non-nullable variable.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
