top-level/impure.nix: fix overlay directory check

pkgs/top-level/impure.nix

@@ -37,26 +37,35 @@ in
   # collections of packages.  These collection of packages are part of the
   # fix-point made by Nixpkgs.
   overlays ? let
-      isDir = path: builtins.pathExists (path + "/.");
+      lib = import ../../lib;
+      inherit (lib.filesystem) pathType;
+      isDir = path: {
+        symlink = builtins.pathExists (toString path + "/");
+        directory = true;
+      }.${pathType path} or false;
       pathOverlays = try (toString <nixpkgs-overlays>) "";
       homeOverlaysFile = homeDir + "/.config/nixpkgs/overlays.nix";
       homeOverlaysDir = homeDir + "/.config/nixpkgs/overlays";
       overlays = path:
         # check if the path is a directory or a file
         if isDir path then
           # it's a directory, so the set of overlays from the directory, ordered lexicographically
-          let content = builtins.readDir path; in
-          map (n: import (path + ("/" + n)))
-            (builtins.filter
-              (n:
-                (builtins.match ".*\\.nix" n != null &&
-                 # ignore Emacs lock files (.#foo.nix)
-                 builtins.match "\\.#.*" n == null) ||
-                builtins.pathExists (path + ("/" + n + "/default.nix")))
-              (builtins.attrNames content))
+          overlaysDir path
         else
           # it's a file, so the result is the contents of the file itself
-          import path;
+          overlaysFile path;
+      overlaysDir = path:
+        let content = builtins.readDir path; in
+        map (n: import (path + ("/" + n)))
+          (builtins.filter
+            (n:
+              (builtins.match ".*\\.nix" n != null &&
+               # ignore Emacs lock files (.#foo.nix)
+               builtins.match "\\.#.*" n == null) ||
+              builtins.pathExists (path + ("/" + n + "/default.nix")))
+            (builtins.attrNames content));
+      overlaysFile = path:
+        import path;
     in
       if pathOverlays != "" && builtins.pathExists pathOverlays then overlays pathOverlays
       else if builtins.pathExists homeOverlaysFile && builtins.pathExists homeOverlaysDir then
@@ -65,13 +74,9 @@ in
           Please remove one of them and try again.
         ''
       else if builtins.pathExists homeOverlaysFile then
-        if isDir homeOverlaysFile then
-          throw (homeOverlaysFile + " should be a file")
-        else overlays homeOverlaysFile
+        overlaysFile homeOverlaysFile
       else if builtins.pathExists homeOverlaysDir then
-        if !(isDir homeOverlaysDir) then
-          throw (homeOverlaysDir + " should be a directory")
-        else overlays homeOverlaysDir
+        overlaysDir homeOverlaysDir
       else []
 
 , crossOverlays ? []