go.crypto/ssh: put version exchange in function

R=golang-dev, dave, jpsugar, agl
CC=golang-dev
https://golang.org/cl/14641044

ssh/client_test.go

@@ -30,5 +30,5 @@ func TestCustomClientVersion(t *testing.T) {
 }
 
 func TestDefaultClientVersion(t *testing.T) {
-	testClientVersion(t, &ClientConfig{}, string(clientVersion))
+	testClientVersion(t, &ClientConfig{}, packageVersion)
 }