Use Go canonical import paths

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

Fixes #29014

pkg/volume/nfs/doc.go

@@ -16,4 +16,4 @@ limitations under the License.
 
 // Package nfs contains the internal representation of network file system
 // (NFS) volumes.
-package nfs
+package nfs // import "k8s.io/kubernetes/pkg/volume/nfs"