Use Go canonical import paths

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

Fixes #29014

contrib/mesos/pkg/executor/doc.go

@@ -18,4 +18,4 @@ limitations under the License.
 Package executor includes a mesos executor, which contains
 a kubelet as its member to manage containers.
 */
-package executor
+package executor // import "k8s.io/kubernetes/contrib/mesos/pkg/executor"