Add direct CAS-to-CAS import for _casbaseddirectory.py
Background
Direct CAS-to-CAS import was removed from !481 (merged) since the same result can be performed via a normal filing system, although we expect direct CAS-to-CAS import to be quicker.
Task description
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Alter CASBasedDirectory.import_files so it recognises an import from another CASBasedDirectory and performs a direct import without using any file systems. -
Test performance with a suitably large dataset (e.g. Freedesktop-SDK)
Acceptance Criteria
CAS-to-CAS import should produce identical results to importing via the file system and should be faster. If it doesn't show any increase in speed we shouldn't merge it.