WIP: Import test for virtual directories

Jim MacArthurrequested to merge
jmac/vdir_import_test into master

Description

Provides a unit test which tests importing of one virtual directory into another.

Changes proposed in this merge request:

  • Alter buildstream/storage/__init__.py so we can more easily import CasBasedDirectory from storage.
  • Add a unit test for CAS-based virtual directory importing.
Edited by Jim MacArthur

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