[config] option to use registered provers from why3 conf without running why3 config detect

To generate the why3find.json config file, I need to either specify the provers manually with why3find config -P provers or to use --detect which will re-generate Why3's config using why3 config detect.

We should add an option (eg why3find config --all) to generate the why3find.json based on the content of Why3's current config, without regenerating it.