Artifact Cache Storage Enhancements

Currently the artifact cache only stores build results, however it would be nice to store the build logs which contributed to the artifact being created, and it will be important to store some metadata related to the artifact.

These should all be stored separately for a given element / cache key in the artifact cache.

New artifact structure can look something like this:

• logs/ <-- location to store logs (will be only one log file at least for now)
• files/ <-- location to store the actual files, the real artifact output
• meta/ <-- location to store element metadata