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 (bst-plugins-experimental#26).
This is required for remote execution and buildbox-run (#719 (closed)).
Part of #1262 (closed).