Automatic merge from submit-queue
pkg/apiserver: split up monolithic package
**Based on** https://github.com/kubernetes/kubernetes/pull/38191
This is a first step to integrate pkg/apiserver with pkg/genericapiserver into a common package structure. For this
- pkg/apiserver is cleaned up from code which does not belong there,
- split up into pkg/apiserver, pkg/apiserver/handlers{,/negotation,/helpers,/errors}
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