Use file tags to generate conversions

This drives conversion generation from file tags like:
  // +conversion-gen=k8s.io/my/internal/version
.. rather than hardcoded lists of packages.

The only net change in generated code can be explained as correct.  Previously
it didn't know that conversion was available.

pkg/apis/batch/v1/doc.go

@@ -15,6 +15,6 @@ limitations under the License.
 */
 
 // +k8s:deepcopy-gen=package,register
+// +k8s:conversion-gen=k8s.io/kubernetes/pkg/apis/batch
 
-// +genconversion=true
 package v1