magic_timestamp is moved into file scope so other classes
can use it.

This is to allow to allow its use by subclasses.

Since access to get_directories is now blocked for some plugins,
and the subclasses of Sandbox do not have configuration defined
by YAML files, they need another way to get at the root directory.

buildstream/storage/Directory.py: New file.
buildstream/storage/_filebaseddirectory.py: New file.
buildstream/_exceptions.py: New VIRTUAL_FS exception source.

This removes _add_directory_to_tarfile since it is now implemented in
_filebaseddirectory.py.

sandbox/_mount.py, sandbox/_sandboxbwrap.py:
  Remove instances of get_directory

This will need more attention when we bring in another virtual
directory backend, however, we've said it is acceptable for the
sandbox itself to access the underlying directory, and this is
the best fix in the meantime.

Abstract directory class and filesystem-backed implementation

See merge request !445

Trying to checkout an element that has no cached artifacts should be and
handled failure. See #447.