[DI] Support optional class for named services

[Haehnchen]

For a valid class name service indexer needs a class attribute, but class is now optional.

services:
    Vendor\Namespace\Class:
        class: Vendor\Namespace\Class
        autowire: true

services:
    Vendor\Namespace\Class:
        autowire: true

symfony/symfony#21133
symfony/symfony@306a060

Notes:
id converts to lower, think of empty syntax

<service id="appbundle\mytestclass" class="AppBundle\MyTestClass"/>
AppBundle\MyTestClass: ~
AppBundle\MyTestClass: {}

[natebrunette]

@Haehnchen I'm having an issue navigating to the service definition when the name is a class name. Is this something that should be working?

[Haehnchen]

@natebrunette if yaml files? fixed by 0bebd36