Support third party autoloaders in Yii::import()

This commit adds support for loading namespaced classes
with third party autoloaders when using `Yii::import()`
and `Yii::createComponent()`. It means that you no longer
need to explicitly set aliases for each and every root
namespace you want to use in your app, which makes working
with composer much easier.

It uses reflection to autoload the class, preserving backwards
compatibility.

Fixes #2642