callPackage: Special treatment for mkPackage

The solution inside makeOverridable was fragile, and did not catch
the potential issue that the new code reports.

It's still not super pretty, but architecturally it won't get better
than this because `by-name/**/package.nix` is tied to `callPackage`.

Alternative solution: require a different file name, so that
`callPackage` can be avoided.

lib/customisation.nix

@@ -170,17 +170,7 @@ rec {
       in
       if isAttrs result then
         result
-        # Overriding `overrideDerivation` and `overrideAttrs` is a hack:
-        # `callPackage` shouldn't have to be aware of those functions and then
-        # fix them up, but it has to because the package arguments aren't part of
-        # the package fixpoint in `mkDerivation`.
-        #
-        # If the original package defines its own `override` method, but not
-        # `overrideDerivation`, we can assume it's a callPackage-aware `mkPackage`
-        # call.
-        # Otherwise, it is a legacy package, for which we still need to perform
-        # the hacky fixup.
-        // optionalAttrs (! (result ? override) || (result ? overrideDerivation)) {
+        // {
           override = overrideArgs;
           overrideDerivation = fdrv: overrideResult (x: overrideDerivation x fdrv);
           ${if result ? overrideAttrs then "overrideAttrs" else null} =
@@ -311,7 +301,22 @@ rec {
 
     in
     if missingArgs == { } then
-      makeOverridable f allArgs
+      if fargs?mkPackage then
+        if fargs != { mkPackage = false; } then
+          throw ''
+            callPackage: A package function that uses `mkPackage` must only have `{ mkPackage }:` as its arguments.
+            Any other dependencies should be taken from the `mkPackage` callback.
+            Otherwise, such dependencies will not be overridable.
+
+            To illustrate, a dependency `foo` can be retrieved with:
+            { mkPackage }: mkPackage ({ layers, foo }: [
+              ...
+            ])
+          ''
+        else
+          f allArgs
+      else
+        makeOverridable f allArgs
     # This needs to be an abort so it can't be caught with `builtins.tryEval`,
     # which is used by nix-env and ofborg to filter out packages that don't evaluate.
     # This way we're forced to fix such errors in Nixpkgs,