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](https://github.com/kubernetes/kubernetes/pull/41747/commits/a0015fd1be6c6a239fc40cc3c04ba5ed7b1ab22e#diff-7a2fbb4371972350ee414c6b88aee1c8) to cut the new repo.