getOptionValueSourceWithGlobals(key) {
    // global overwrites local, like optsWithGlobals
    let source;
    this._getCommandAndAncestors().forEach((cmd) => {
      if (cmd.getOptionValueSource(key) !== undefined) {
        source = cmd.getOptionValueSource(key);
      }
    });
    return source;
  }
Get source of option value. See also .optsWithGlobals().
Expected values are default | config | env | cli | implied

@param {string} key
@return {string}