Enable setting an object element's value from the override file

Currently, there is no way to directly set a field inside of an object type variable. That is to say, any time a pack user wants to override a field in an object they must provide the entire object value.

```golang
// NOTE: This implementation assumes a single element variable value,
// which specifically breaks setting a sub-element value. The more
// correct way is to perform the traversal based on the provided root
// pack
```