To try it (without installing Catala):
```shell-session
$ make plugins
$ export CATALA_PLUGINS=_build/default/compiler/plugins
$ dune exec -- catala lazy examples/aides_logement/tests/tests_calcul_apl_locatif.catala_fr -s Exemple2
```
Keep in mind that this is a work-in-progress prototype :)
Fixes#378
- the plugins are compiled as libraries rather than with `executable`, so that
dune is able to install them
- they get installed to `lib/catala/plugins/<plugin-name>/<plugin-name>.cmxs`
- the lookup for plugins is now recursive to cope with the plugin subdirectories
in the point above
It's not expected to stay that way forever, but some additional effort will be required for them to preserve (or restore) types; until then, be safe and don't forward possibly incorrect information.