I see. It seems the Fortran rewrite might be more difficult. So I think for now we should simply depend on fypp to be in PATH and fpm will call it automatically on .fypp files and take care of everything else, so you just do fpm build or fpm test and things will re-generate / recompile automatically under the hood.
In a separate issue we can brainstorm what it would take to rewrite into Fortran. I proposed this to our monthly Fortran call: Fortran Monthly Call: July 2021 - #5 by certik