Seems like wedging this into `add_external_plugin` is a bit awkward... the SHA is meaningless, the inclusion of `https://` is meaningless but (if I'm reading correctly) required. What I'd rather see is a separate `dev_external_plugin` function that takes a single argument of a filepath. And, if possible, the build should scan the `externals` folder on each build, automatically calling `dev_external_plugin` for each sub-folder; then you could just clone a repo into the externals folder and run make, no need to edit the config at all.