Cache artifacts with virtual directories instead of filesystem.

Description

_cache_artifacts currently extracts the sandbox's virtual directory (which is often CAS-based) to a file system then creates a new CAS-based directory to send to the artifact cache. This changes it to do a direct CAS-to-CAS import, with a few parts still using filesystem (logs and metadata) being imported into that instead.

Changes proposed in this merge request:

• Use virtual directories to compose the artifact.
• Add get_size to the Virtual Directory API.
• Correct a fault in CasBasedDirectory - make absolute symbolic links into relative ones on import.

This merge request, when approved, will close: #787 (closed)

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