DRA: bump API v1alpha2 -> v1alpha3

This is in preparation for revamping the resource.k8s.io completely. Because
there will be no support for transitioning from v1alpha2 to v1alpha3, the
roundtrip test data for that API in 1.29 and 1.30 gets removed.

Repeating the version in the import name of the API packages is not really
required. It was done for a while to support simpler grepping for usage of
alpha APIs, but there are better ways for that now. So during this transition,
"resourceapi" gets used instead of "resourcev1alpha3" and the version gets
dropped from informer and lister imports. The advantage is that the next bump
to v1beta1 will affect fewer source code lines.

Only source code where the version really matters (like API registration)
retains the versioned import.

Kubernetes-commit: b51d68bb87ba4fa47eb760f8a5e0baf9cf7f5b53

Author: pohly