php: Make buildEnv recursive + take extension deps into account

A slight rewrite of buildEnv which:

1. Makes buildEnv recursively add itself to its output, so that it can
   be accessed from any php derivation.

2. Orders the extension text strings according to their internalDeps
   attribute - dependencies have to be put before dependants in the
   php.ini or they will fail to load due to missing symbols.

pkgs/top-level/php-packages.nix

@@ -717,7 +717,7 @@ let
       , doCheck ? true
       , ...
     }: stdenv.mkDerivation {
-      pname = "php-ext-${name}";
+      pname = "php-${name}";
 
       inherit (php) version src;
       sourceRoot = "php-${php.version}/ext/${name}";
@@ -746,7 +746,7 @@ let
       checkPhase = "echo n | make test";
       installPhase = ''
         mkdir -p $out/lib/php/extensions
-        cp modules/${name}.so $out/lib/php/extensions/ext-${name}.so
+        cp modules/${name}.so $out/lib/php/extensions/${name}.so
       '';
     };