* Added ControllerFeature and ControllerFeatureProvider to perform controller discovery.
* Changed controller discovery to use application parts.
* Changed ControllerActionDescriptorProvider to make use of Application parts.
* Simplified AddControllerAsServices to not accept any parameter and perform
controller discovery through the ApplicationPartManager in the IMvcBuilder
and IMvcCoreBuilder. Assemblies should be added to the ApplicationPartManager
in order to discover controllers in them.
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