Automatic merge from submit-queue
let client-gen understand vendor
This is extracted from #44784, where we move external api types to k8s.io/api. After the move, the types will locate at vendor/k8s.io/api/xxx. However, gengo index the parsed package using the import path, which is stripped of the vendor/ prefix, so we'll need to strip the vendor/ prefix as necessary in client-gen.
This PR doesn't produce any change to the generated clientset yet since all types are still in the kubernetes repo.
8214f7bb29