remove special clientrepo code from main repository gcp plugin

rename plugin/pkg/client/auth/plugins.go package name to auth

add the plugin import line in client-gen

update import_known_versions for release_1_5 clientset

change copy.sh

plugin/pkg/client/auth/plugins.go

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-package plugins
+package auth
 
 import (
 	// Initialize all known client auth plugins.