Clerk: allow to include non-yet-existing directories

Useful when you have wide `-I` options that not all targets may depend on.

build_system/clerk_driver.ml

@@ -53,7 +53,7 @@ module Cli = struct
   let include_dirs =
     Arg.(
       value
-      & opt_all dir []
+      & opt_all string []
       & info ["I"; "include"] ~docv:"DIR"
           ~doc:
             "Make modules from the given directory available from everywhere.")

compiler/catala_utils/file.ml

@@ -213,14 +213,15 @@ module Tree = struct
 
   let rec build path =
     lazy
-      (Array.fold_left
+      (let entries = try Sys.readdir path with Sys_error _ -> [||] in
+       Array.fold_left
          (fun m f ->
            let path = path / f in
            match Sys.is_directory path with
            | true -> Map.add f (path, D (build path)) m
            | false -> Map.add f (path, F) m
            | exception Sys_error _ -> m)
-         Map.empty (Sys.readdir path))
+         Map.empty entries)
 
   let subtree t path =
     let rec aux t = function