haskell.lib.packagesFromDirectory: only `.nix` files

This changes `haskell.lib.packagesFromDirectory` to ignore non-Nix
files so that it doesn't attempt to run `callPackage` on, say, a
`.md` file in the same directory.

pkgs/development/haskell-modules/lib/compose.nix

···
 
     self: super:
       let
-        haskellPaths = builtins.attrNames (builtins.readDir directory);
+        haskellPaths =
+          lib.filter (lib.hasSuffix ".nix")
+            (builtins.attrNames (builtins.readDir directory));
 
         toKeyVal = file: {
           name  = builtins.replaceStrings [ ".nix" ] [ "" ] file;