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Types that we want to work using apimachinery typically go into
pkg/apis/X, and the validation goes into pkg/apis/X/validation.  We then
add versions into e.g. pkg/apis/X/v1alpha1, but this feels premature at
the moment.

Changing this later is annoyingly difficult, especially in terms of
validation and dependencies.

We will want the apimachinery, so that we can configure from a
configuration file that is versioned.  Hopefully dns-controller won't
end up so complicated that we will require it, but I think there is also
value in following the "standard" patterns for controllers that are
emerging from e.g. ingress.

For a fairly simple example of an API, please consult
https://github.com/kubernetes/kubernetes/tree/master/pkg/apis/certificates