Zend Framework composer autoload

I have PHP project with the following vendors directory structure:

vendor
  zendframework
    zendframework
      library
       Zend
         ...
         ...
         Cache
         ...
         ...

When I put the following line into autoload_namespaces.php everything is ok:

'Zend\\Cache' => $vendorDir . '/zendframework/zendframework/library/',

but this line doesn't work:

'Zend\\Cache' => $vendorDir . '/zendframework/zendframework/library/Zend/Cache/',

The error is:

Fatal error: Class 'Zend\Cache\Storage\Adapter\MemcachedOptions' not found

What's wrong with my code? How can I load only Cache module from zend framework? Why does it work if I specify whole library folder?