Element state / cache key handling

Cache keys are currently calculated on demand as they cannot always be calculated early on. E.g., tracking affects cache keys and calculating strong cache keys with a non-strict build plan requires extraction of metadata from possibly downloaded dependencies.

There is a caching mechanism for the calculated cache keys, however, there are still situations where the same cache key is calculated multiple times. This can incur a performance overhead.

Related issues #149 (closed) and #167 (closed).