Use Go canonical import paths

Add canonical imports only in existing doc.go files.
https://golang.org/doc/go1.4#canonicalimports

Fixes #29014

plugin/pkg/auth/authenticator/doc.go

@@ -15,4 +15,4 @@ limitations under the License.
 */
 
 // Package authenticator contains implementations for pkg/auth/authenticator interfaces
-package authenticator
+package authenticator // import "k8s.io/kubernetes/plugin/pkg/auth/authenticator"