code: function getFileExtnameAssociatedProcessor(processors) {
      let result = Object.keys(processors).reduce(
          (lastValue, processorName) => {
              let processor = processors[processorName];
              let extnames = processor.extnames || [];
              if (!Array.isArray(extnames)) {
                  extnames = [extnames];
              }
              addFileExtnameAssociatedProcessor(extnames, processorName, lastValue);
              return lastValue;
          },
          {}
      );
  
      Object.keys(result).forEach(
          k => sortDefaultProcessors(result[k], processors)
      );
      return result;
  }
docs: Get the map of the file extname with processor name.
  
  @inner
  @param {Object} processors the builtin processors, the key is processor name,
  the value is the processor config information.
  @return {Object}