Require virtual directory support for element plugins

This drops support for element plugins that use Sandbox.get_directory() instead of Sandbox.get_virtual_directory(). All plugins in this repo have been ported a long time ago. All plugins in bst-plugins-experimental have been ported either in a branch or already in master (https://gitlab.com/BuildStream/bst-plugins-experimental/-/issues/26).

This is required for remote execution and buildbox-run (#719 (closed)).

Part of #1262 (closed).