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.

NB Could this be done just with get_virtual_directory and .external_directory?

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

Add get_virtual_directory and get_virtual_toplevel_directory().

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