libpng12: migrate to pkgs/by-name

This moves libpng12 to the by-name directory layout:

  pkgs/by-name/li/libpng/12.nix → pkgs/by-name/li/libpng12/package.nix

The file is renamed to package.nix to follow by-name conventions, and the
fixed-output hash is updated to the modern `hash =` form.

The old libpng12 entry in all-packages.nix is removed, since by-name
packages are now auto-discovered.

pkgs/by-name/li/libpng12/package.nix

@@ -14,7 +14,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   src = fetchurl {
     url = "mirror://sourceforge/libpng/libpng-${finalAttrs.version}.tar.xz";
-    sha256 = "1izw9ybm27llk8531w6h4jp4rk2rxy2s9vil16nwik5dp0amyqxl";
+    hash = "sha256-tGNfFbitzMitCTTupIXvWcxMriTQ8DAKmpQeUZdP/Mc=";
   };
 
   postPatch = lib.optionalString stdenv.hostPlatform.isDarwin ''

pkgs/top-level/all-packages.nix

@@ -7151,8 +7151,6 @@ with pkgs;
   libpeas = callPackage ../development/libraries/libpeas { };
   libpeas2 = callPackage ../development/libraries/libpeas/2.x.nix { };
 
-  libpng12 = callPackage ../development/libraries/libpng/12.nix { };
-
   inherit
     (callPackages ../development/libraries/prometheus-client-c {
       stdenv = gccStdenv; # Required for darwin