lib.customisation: uncurry makeScopeWithSplicing

Deeply-curried functions are pretty error-prone in untyped languages
like Nix.  This is a particularly bad case because
`top-level/splice.nix` *also* declares a makeScopeWithSplicing, but
it takes *two fewer arguments*.

Let's switch to attrset-passing form, to provide some minimal level
of sanity-checking.

pkgs/development/interpreters/python/passthrufun.nix

@@ -60,12 +60,10 @@
         overrides
       ]);
       aliases = self: super: lib.optionalAttrs config.allowAliases (import ../../../top-level/python-aliases.nix lib self super);
-    in makeScopeWithSplicing
-      otherSplices
-      keep
-      extra
-      (lib.extends (lib.composeExtensions aliases extensions) keep))
-    {
+    in makeScopeWithSplicing {
+      inherit otherSplices keep extra;
+      f = lib.extends (lib.composeExtensions aliases extensions) keep;
+    }) {
       overrides = packageOverrides;
       python = self;
     });