use Composer autoloader not OC for non-Composer 3rdparty (#9643)

Composer's autoloader is rather better than the OwnCloud autoloader's handling of non-OC classes. Plus we can rely on upstream Composer to maintain it and not worry about it ourselves.

With this change, we drop the bits of OwnCloud's autoloader that handled non-OC classes, and register the classes that were being handled by that code with Composer's autoloader instead. As these dependencies are converted to actually being managed by Composer, the explicit registrations can be dropped as they won't be needed any more.

The movement of `OC_Util::isSetLocaleWorking();` below the composer autoloader registration is not an accident, it is necessary, because that call won't work unless autoloading of Patchwork works, so it has to come after composer's autoloader is registered.

This is my approach to https://github.com/owncloud/core/issues/9643 . It's rather simpler than @th3fallen 's https://github.com/owncloud/core/pull/9714 .

Please consider this contribution to be MIT-licensed, that is:

Copyright (c) 2014 Adam Williamson

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Probably needs an else case here throwing a fatal error since composer autoloader is now a mandatory dependency.