Automatic merge from submit-queue
Move api helpers.go to a subpackage
Part of https://github.com/kubernetes/kubernetes/issues/44065.
This PR moves the pkg/api/helpers.go to its own subpackage. It's mostly a mechanic move, except that
* I removed ConversionError in helpers.go, it's not used by anyone
* I moved the 3 methods of Taint and Toleration to pkg/api/methods.go, and left a TODO saying refactoring these methods to functions.
I'll send a few more PRs to make the k8s.io/kubernetes/pkg/api package only contains the code we want in the k8s.io/api repo, then we can run a [script](a0015fd1be (diff-7a2fbb4371972350ee414c6b88aee1c8)) to cut the new repo.