MDL-49329 admin: Archive plugin code before removing it from dirroot

MDL-49329 admin: Archive plugin code before removing it from dirroot

This should allow the admin to revert the upgrade of existing plugins,
such when the dependency chain leads to a dead-end. Additionally, we
archive (as a last-chance copy) the to-be-installed plugins when
cancelling their installation. This is mainly for developers who could
otherwise loose their code. For the same reason, plugins are being
archived upon uninstallation, too.