gnuplot: migrate to by-name, switch to finalAttrs, and remove Qt-specific mkDerivation

This refactor performs a mechanical migration of gnuplot to the by-name
layout and updates the derivation to the modern finalAttrs style.

Key changes:

* Move gnuplot from pkgs/tools/graphics/gnuplot/ to pkgs/by-name/gn/gnuplot/
  and rename set-gdfontpath-from-fontconfig.sh accordingly.

* Replace the old `rec { ... }` form with `stdenv.mkDerivation (finalAttrs: { ... })`
  following current Nixpkgs conventions.

* Remove the unused mkDerivation argument and stop switching the derivation
  function based on `withQt`. gnuplot does not require libsForQt5.mkDerivation;
  Qt support is handled entirely through inputs and configure flags.

* Replace qttools/qtbase/qtsvg with libsForQt5.qttools/qtbase/qtsvg when
  `withQt = true`.

* Convert `${pname}-${version}` to `${finalAttrs.version}` and update
  `sha256` → `hash`.

* Add by-name variants gnuplot_qt and gnuplot_aquaterm using override
  wrappers.

* Remove the old top-level aliases for gnuplot, gnuplot_qt, and
  gnuplot_aquaterm from all-packages.nix; by-name exposure now handles them.

This change is purely structural and does not modify gnuplot’s build
behaviour beyond adopting the standard finalAttrs pattern.

pkgs/by-name/gn/gnuplot/package.nix

@@ -28,22 +28,19 @@
   gnused,
   coreutils,
   withQt ? false,
-  qttools,
-  wrapQtAppsHook,
-  qtbase,
-  qtsvg,
+  qt5,
 }:
 
 let
   withX = !aquaterm && !stdenv.hostPlatform.isDarwin;
 in
-stdenv.mkDerivation rec {
+stdenv.mkDerivation (finalAttrs: {
   pname = "gnuplot";
   version = "6.0.4";
 
   src = fetchurl {
-    url = "mirror://sourceforge/gnuplot/gnuplot-${version}.tar.gz";
-    sha256 = "sha256-RY2UdpYl5z1fYjJQD0nLrcsrGDOA1D0iZqD5cBrrnFs=";
+    url = "mirror://sourceforge/gnuplot/gnuplot-${finalAttrs.version}.tar.gz";
+    hash = "sha256-RY2UdpYl5z1fYjJQD0nLrcsrGDOA1D0iZqD5cBrrnFs=";
   };
 
   nativeBuildInputs = [
@@ -52,8 +49,8 @@ stdenv.mkDerivation rec {
     texinfo
   ]
   ++ lib.optionals withQt [
-    qttools
-    wrapQtAppsHook
+    qt5.qttools
+    qt5.wrapQtAppsHook
   ];
 
   buildInputs = [
@@ -74,8 +71,8 @@ stdenv.mkDerivation rec {
     libxaw
   ]
   ++ lib.optionals withQt [
-    qtbase
-    qtsvg
+    qt5.qtbase
+    qt5.qtsvg
   ]
   ++ lib.optional withWxGTK wxwidgets_3_2;
 
@@ -131,4 +128,4 @@ stdenv.mkDerivation rec {
     maintainers = [ ];
     mainProgram = "gnuplot";
   };
-}
+})

pkgs/by-name/gn/gnuplot_aquaterm/package.nix

@@ -0,0 +1,7 @@
+{
+  gnuplot,
+}:
+
+gnuplot.override {
+  aquaterm = true;
+}

pkgs/by-name/gn/gnuplot_qt/package.nix

@@ -0,0 +1,7 @@
+{
+  gnuplot,
+}:
+
+gnuplot.override {
+  withQt = true;
+}

pkgs/top-level/all-packages.nix

@@ -2401,13 +2401,6 @@ with pkgs;
   };
   gnupg = gnupg24;
 
-  gnuplot = libsForQt5.callPackage ../tools/graphics/gnuplot { };
-
-  gnuplot_qt = gnuplot.override { withQt = true; };
-
-  # must have AquaTerm installed separately
-  gnuplot_aquaterm = gnuplot.override { aquaterm = true; };
-
   gnused = callPackage ../tools/text/gnused { };
 
   gnutar = callPackage ../tools/archivers/gnutar { };