Bring back Fs_memo

Signed-off-by: Marek Kubica <[email protected]>

Author: Leonidas-from-XIV

src/dune_rules/pkg_rules.ml

@@ -1494,10 +1494,10 @@ end = struct
             in
             resolve db dep_loc dep_pkg_digest package_universe)
       and+ files_dir =
-        let+ lock_dir =
+        let* lock_dir =
           Package_universe.lock_dir_path package_universe >>| Option.value_exn
         in
-        let files_dir =
+        let+ files_dir =
           let module Pkg = Dune_pkg.Lock_dir.Pkg in
           (* TODO(steve): simplify this once portable lockdirs become the
              default. This logic currently handles both the cases where
@@ -1509,15 +1509,17 @@ end = struct
           let path_with_version =
             Pkg.source_files_dir info.name (Some info.version) ~lock_dir
           in
-          let path_with_version_exists =
-            Path.Untracked.exists (Path.source path_with_version)
+          let* path_with_version_exists =
+            Fs_memo.dir_exists (Path.Outside_build_dir.In_source_dir path_with_version)
           in
           match path_with_version_exists with
-          | true -> Some (Pkg.files_dir info.name (Some info.version) ~lock_dir)
+          | true ->
+            Memo.return @@ Some (Pkg.files_dir info.name (Some info.version) ~lock_dir)
           | false ->
             let path_without_version = Pkg.source_files_dir info.name None ~lock_dir in
-            let path_without_version_exists =
-              Path.Untracked.exists (Path.source path_without_version)
+            let+ path_without_version_exists =
+              Fs_memo.dir_exists
+                (Path.Outside_build_dir.In_source_dir path_without_version)
             in
             (match path_without_version_exists with
              | true -> Some (Pkg.files_dir info.name None ~lock_dir)