Install Catala plugins

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

compiler/catala_utils/cli.ml

@@ -172,7 +172,7 @@ let plugins_dirs =
   let default =
     let ( / ) = Filename.concat in
     [
-      Sys.executable_name
+      Filename.dirname Sys.executable_name
       / Filename.parent_dir_name
       / "lib"
       / "catala"

compiler/driver.ml

@@ -73,7 +73,15 @@ let driver source_file (options : Cli.options) : int =
         try `Plugin (Plugin.find s)
         with Not_found ->
           Errors.raise_error
-            "The selected backend (%s) is not supported by Catala" backend)
+            "The selected backend (%s) is not supported by Catala, nor was a \
+             plugin by this name found under %a"
+            backend
+            (Format.pp_print_list
+               ~pp_sep:(fun ppf () -> Format.fprintf ppf "@ or @ ")
+               (fun ppf dir ->
+                 Format.pp_print_string ppf
+                   (try Unix.readlink dir with _ -> dir)))
+            options.plugins_dirs)
     in
     let prgm =
       Surface.Parser_driver.parse_top_level_file source_file language

compiler/plugin.ml

@@ -58,12 +58,16 @@ let load_file f =
     Errors.format_warning "Could not load plugin %S: %s" f
       (Printexc.to_string e)
 
-let load_dir d =
+let rec load_dir d =
   let dynlink_exts =
     if Dynlink.is_native then [".cmxs"] else [".cmo"; ".cma"]
   in
   Array.iter
     (fun f ->
-      if List.exists (Filename.check_suffix f) dynlink_exts then
+      if f.[0] = '.' then ()
-        load_file (Filename.concat d f))
+      else
+        let f = Filename.concat d f in
+        if Sys.is_directory f then load_dir f
+        else if List.exists (Filename.check_suffix f) dynlink_exts then
+          load_file f)
     (Sys.readdir d)

compiler/plugins/dune

@@ -1,18 +1,22 @@
-(executable
+(library
  (name python)
- (modes plugin)
+ (public_name catala.plugins.python)
+ (synopsis
+  "Demonstration Catala plugin that reproduces the behaviour of the built-in python backend")
  (modules python)
  (libraries catala.driver))
 
-(executable
+(library
  (name api_web)
- (modes plugin)
+ (public_name catala.plugins.api_web)
+ (synopsis "Catala plugin for interaction with a web interface")
  (modules api_web)
  (libraries catala.driver))
 
-(executable
+(library
  (name json_schema)
- (modes plugin)
+ (public_name catala.plugins.json_schema)
+ (synopsis "Catala plugin generating JSON schemas useful to build web-forms")
  (modules json_schema)
  (libraries catala.driver))