libpng: migrate to pkgs/by-name

This moves libpng to the by-name directory layout:

  pkgs/development/libraries/libpng → pkgs/by-name/li/libpng

The package is renamed to package.nix.

The previous Darwin-specific stdenv override (using darwin.bootstrapStdenv
to avoid xcbuild recursion) is preserved by introducing a local stdenv'
binding and using it for mkDerivation. This keeps the bootstrap behavior
identical to the old location.

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

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

@@ -5,6 +5,7 @@
   zlib,
   apngSupport ? true,
   testers,
+  darwin,
 }:
 
 assert zlib != null;
@@ -16,9 +17,9 @@ let
     hash = "sha256-aH3cDHyxKKPqWOFZtRKSUlN8J+3gwyqT8R8DEn8MAWU=";
   };
   whenPatched = lib.optionalString apngSupport;
-
+  stdenv' = if stdenv.hostPlatform.isDarwin then darwin.bootstrapStdenv else stdenv;
 in
-stdenv.mkDerivation (finalAttrs: {
+stdenv'.mkDerivation (finalAttrs: {
   pname = "libpng" + whenPatched "-apng";
   version = "1.6.53";
 

pkgs/top-level/all-packages.nix

@@ -7151,13 +7151,6 @@ with pkgs;
   libpeas = callPackage ../development/libraries/libpeas { };
   libpeas2 = callPackage ../development/libraries/libpeas/2.x.nix { };
 
-  libpng = callPackage ../development/libraries/libpng {
-    stdenv =
-      # libpng is a dependency of xcbuild. Avoid an infinite recursion by using a bootstrap stdenv
-      # that does not propagate xcrun.
-      if stdenv.hostPlatform.isDarwin then darwin.bootstrapStdenv else stdenv;
-  };
-
   libpng12 = callPackage ../development/libraries/libpng/12.nix { };
 
   inherit