builder: do not parse the same repository file name twice

When parsing the repository configuration files, track the file names
parsed, and in case of same file name in multiple location consider only
the one in the directory coming before others in the prioritised list of
paths.

This way it is possible to "shadow" a configuration file in a system
path with one with the same name in a local directory, for example.

builder/sources.ml

@@ -28,6 +28,8 @@ type source = {
   gpgkey : string option;
 }
 
+module StringSet = Set.Make (String)
+
 let parse_conf ~prog ~debug file =
   if debug then (
     eprintf (f_"%s: trying to read %s\n") prog file;
@@ -96,15 +98,23 @@ let read_sources ~prog ~debug =
     | None -> dirs
     | Some dir -> dir :: dirs in
   let dirs = List.map (fun x -> x // "repos.d") dirs in
-  List.fold_right (
-    fun dir acc ->
+  let fnseen = ref StringSet.empty in
+  List.fold_left (
+    fun acc dir ->
       let files =
         try List.filter filter_filenames (Array.to_list (Sys.readdir dir))
         with Sys_error _ -> [] in
-      let files = List.map (fun x -> dir // x) files in
+      let files = List.filter (fun x -> StringSet.mem x !fnseen <> true) files in
       List.fold_left (
         fun acc file ->
-          try merge_sources acc (parse_conf ~prog ~debug file) with
+          try (
+            let s = merge_sources acc (parse_conf ~prog ~debug (dir // file)) in
+            (* Add the current file name to the set only if its parsing
+             * was successful.
+             *)
+            fnseen := StringSet.add file !fnseen;
+            s
+          ) with
           | Unix_error (code, fname, _) ->
             if debug then (
               eprintf (f_"%s: file error: %s: %s\n") prog fname (error_message code)
@@ -116,4 +126,4 @@ let read_sources ~prog ~debug =
             );
             acc
       ) acc files
-  ) dirs []
+  ) [] dirs