lib/classes/Variables.js/Variables/getValueFromSource
class Variables:  getValueFromSource(variableString) {
    let valueToPopulate;
    if (variableString.match(this.envRefSyntax)) {
      valueToPopulate = this.getValueFromEnv(variableString);
    } else if (variableString.match(this.optRefSyntax)) {
      valueToPopulate = this.getValueFromOptions(variableString);
    } else if (variableString.match(this.selfRefSyntax)) {
      valueToPopulate = this.getValueFromSelf(variableString);
    } else if (variableString.match(this.fileRefSyntax)) {
      valueToPopulate = this.getValueFromFile(variableString);
    } else {
      const errorMessage = [
        `Invalid variable reference syntax for variable ${variableString}.`,
        ' You can only reference env vars, options, & files.',
        ' You can check our docs for more info.',
      ].join('');
      throw new this.serverless.classes.Error(errorMessage);
    }
    return valueToPopulate;
  }