We cant do that since we run carton as an external process (see https://github.com/rejeep/carton/blob/v0.4-wip/features/step-definitions/carton-steps.el#L15-L33). We do require carton in `env.el`, but we really shouldn't. I made a pull request that removes it (see https://github.com/rejeep/carton/pull/61). So in the Ecukes tests, we don't have access to carton, hence we cannot modify `carton-source-mapping`.