flattenReferencesGraph: avoid lib.fileset

I am not sure `nix-gitignore.gitignoreSource` fares any better
with chroot stores, but so far it’s always been `lib.fileset`
that has caused issues with the installer ISO, and a blanket ban on
`lib.fileset` seems like the simplest remedy unless it turns out that
one of the other source filtering helpers causes issues.

This could probably be omitted entirely as it’s just an aid for
development convenience, anyway.

This reverts commit ba5a5fac7b.
This reverts commit 9ae0f77577.

pkgs/by-name/fl/flattenReferencesGraph/package.nix

@@ -1,25 +1,19 @@
 {
   callPackage,
-  lib,
+  nix-gitignore,
   python3Packages,
 }:
 let
-  inherit (lib) fileset;
   helpers = callPackage ./helpers.nix { };
   pythonPackages = python3Packages;
+
 in
 pythonPackages.buildPythonApplication {
   version = "0.1.0";
   pname = "flatten-references-graph";
 
-  src = fileset.toSource {
-    root = ./src;
-    fileset = fileset.unions [
-      ./src/.flake8
-      ./src/flatten_references_graph
-      ./src/setup.py
-    ];
-  };
+  # Note: this uses only ./src/.gitignore
+  src = nix-gitignore.gitignoreSource [ ] ./src;
 
   propagatedBuildInputs = with pythonPackages; [
     igraph