使用composer安装后，Symfony中没有调用类

I have this directory structure:

symfonyproject/src/mybundle/composer.json

Inside of this bundle I did a:

composer require sabre/xml

It got installed fine, just like every other installed packaged, however it won't load.

use Sabre;

...

$writer = new Sabre\Xml\Writer();

This gives me this error:

Attempted to load class "Writer" from namespace "Sabre\Xml". Did you forget a "use" statement for another namespace?

I have checked: symfonyproject/src/mybundle/vendor/composer/autoloade_psr4

And the parameters are there:

'Sabre\\Xml\\' => array($vendorDir . '/sabre/xml/lib'),
'Sabre\\Uri\\' => array($vendorDir . '/sabre/uri/lib'),

I am not sure what am I doing wrong here. Other packages work. Any help?