Remove kubectl's dependence on schema file in pkg/api/validation.

**What this PR does / why we need it**:

Makes functions in validation/schema.go private to kubectl,
further isolating kubectl.

**Which issue this PR fixes**

Part of a series of PRs to address kubernetes/community#598

**Release note**:
```release-note
NONE
```

hack/make-rules/test-cmd-util.sh

@@ -687,7 +687,7 @@ run_pod_tests() {
   kube::test::get_object_assert pods "{{range.items}}{{$id_field}}:{{end}}" 'valid-pod:'
 
   ## Patch can modify a local object
-  kubectl patch --local -f pkg/api/validation/testdata/v1/validPod.yaml --patch='{"spec": {"restartPolicy":"Never"}}' -o jsonpath='{.spec.restartPolicy}' | grep -q "Never"
+  kubectl patch --local -f pkg/kubectl/validation/testdata/v1/validPod.yaml --patch='{"spec": {"restartPolicy":"Never"}}' -o jsonpath='{.spec.restartPolicy}' | grep -q "Never"
 
   ## Patch pod can change image
   # Command