Make unnecessary hpa public funcs private

Previously
`pkg.controller.podautoscaler.UnsafeConvertToVersion` was
exported. However, it was never used outside of the `podautoscaler`
package. Make it private to prevent confusion.

Additionally, move the two private functions in `horizontal.go` to be
with the other private functions at the bottom of the file - imho its
more readable than having them directly at the top of the file, before
the public type and function definitions.

pkg/controller/podautoscaler/horizontal_test.go

@@ -232,7 +232,7 @@ func (tc *testCase) prepareTestClient(t *testing.T) (*fake.Clientset, *metricsfa
 		}
 
 		// and... convert to autoscaling v1 to return the right type
-		objv1, err := UnsafeConvertToVersionVia(obj, autoscalingv1.SchemeGroupVersion)
+		objv1, err := unsafeConvertToVersionVia(obj, autoscalingv1.SchemeGroupVersion)
 		if err != nil {
 			return true, nil, err
 		}