refactor: use [Filename.t] instead of [string] (#6984)

Signed-off-by: Rudi Grinberg <[email protected]>

src/dune_engine/cram_test.mli

@@ -13,6 +13,6 @@ val dyn_of_t : t -> Dyn.t
 
 val name : t -> string
 
-val fname_in_dir_test : string
+val fname_in_dir_test : Filename.t
 
 val script : t -> Path.Source.t

src/dune_engine/dune_project.mli

@@ -153,7 +153,7 @@ val anonymous :
   -> t
 
 (** "dune-project" *)
-val filename : string
+val filename : Filename.t
 
 (** Default language version to use for projects that don't have a
     [dune-project] file. The default value is the latest version of the dune

src/dune_engine/source_tree.mli

@@ -3,7 +3,7 @@
 open Import
 
 module Dune_file : sig
-  val fname : string
+  val fname : Filename.t
 
   val alternative_fname : string
 

src/dune_rules/workspace.mli

@@ -101,7 +101,7 @@ module Clflags : sig
 end
 
 (** Default name of workspace files *)
-val filename : string
+val filename : Filename.t
 
 val workspace : unit -> t Memo.t
 

src/dune_sexp/decoder.mli

@@ -187,7 +187,7 @@ val junk_everything : (unit, _) parser
 val plain_string : (loc:Loc.t -> string -> 'a) -> 'a t
 
 (** A valid filename, i.e. a string other than "." or ".." *)
-val filename : string t
+val filename : Filename.t t
 
 (** A relative filename *)
 val relative_file : string t