Currently the HCL2 upgrade command generates a valid variable block for all variables
within the JSON variables property. However JSON templates in Packer support variable
interpolation for input variables defined within a variable definition file without
it being declared within the variables property. When upgrading a JSON template to HCL2
the user variable reference gets converted to var.<var_name>. The upgraded template ultimately
fails on validation or build execution with undefined variable errors, with this change the upgrade
command will now create a variable block for all input variables referenced within the build template
to ensure all required variables been defined.