Read from {mdl}.json file instead of yaml in create package

[chrispcampbell]

I found this code and comment in the create package in step-config.ts:

  // Read `build/{mdl}_vars.yaml`
  // TODO: For now the printVarList code only outputs txt and yaml files; we'll use the
  // yaml file for now, but that means we have a dependency on the `yaml` package.  Once
  // we change the `compile` package to output JSON, we'll need to change this code.
  const varsYamlFile = joinPath(buildDir, `${mdlName}_vars.yaml`)
  const varsYamlContent = await readFile(varsYamlFile, 'utf8')
  const varObjs = yaml.parse(varsYamlContent)

We are about to have the JSON file available as an alternative to the YAML one (see #105). Once those changes land, we can update the above code to read from {mdl}.json, and we can drop the yaml package dependency.