Make parent dir available in fileset.fileFilter

Currently, fileFilter only allows filtering based based on a files base
name + file type.

This is a bit limiting if you want to include files based on the name of
the directory they reside in.

I bumped in to this when using fileFilter to make a minimal set of files
to make Cargo happy in a Rust workspace - specifically, I needed to pull
in any `.rs` file that lived under a `bin/`, `examples/`, or `benches/`
folder before Cargo would stop complaining. (Note that I am trying to
avoid pulling in all rust files - I'm working in a large Rust workspace
but trying to package a single small member of the workspace).

```

lib/fileset/default.nix

@@ -668,6 +668,8 @@ in {
 
       - `name` (String): The name of the file
 
+      - `dir` (Path): The path of the directory containing the file.
+
       - `type` (String, one of `"regular"`, `"symlink"` or `"unknown"`): The type of the file.
         This matches result of calling [`builtins.readFileType`](https://nixos.org/manual/nix/stable/language/builtins.html#builtins-readFileType) on the file's path.
 

lib/fileset/internal.nix

@@ -825,11 +825,11 @@ rec {
   _fileFilter = predicate: root:
     let
       # Check the predicate for a single file
-      # Type: String -> String -> filesetTree
-      fromFile = name: type:
+      # Type: Path -> String -> String -> filesetTree
+      fromFile = dir: name: type:
         if
           predicate {
-            inherit name type;
+            inherit dir name type;
             hasExt = ext: hasSuffix ".${ext}" name;
 
             # To ensure forwards compatibility with more arguments being added in the future,
@@ -848,7 +848,7 @@ rec {
           if type == "directory" then
             fromDir (path + "/${name}")
           else
-            fromFile name type
+            fromFile path name type
         ) (readDir path);
 
       rootType = pathType root;