Use Go canonical import paths

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

Fixes #29014

pkg/api/validation/doc.go

@@ -16,4 +16,4 @@ limitations under the License.
 
 // Package validation has functions for validating the correctness of api
 // objects and explaining what is wrong with them when they aren't valid.
-package validation
+package validation // import "k8s.io/kubernetes/pkg/api/validation"