php.packages: Make packages overridable

This will allow patching extensions without having to manually having to patch all occurrences in the dependency tree.

pkgs/development/interpreters/php/generic.nix

@@ -13,6 +13,7 @@ let
     , version
     , sha256
     , extraPatches ? []
+    , packageOverrides ? (final: prev: {})
 
     # Sapi flags
     , cgiSupport ? true
@@ -50,7 +51,7 @@ let
 
               php-packages = (callPackage ../../../top-level/php-packages.nix {
                 phpPackage = phpWithExtensions;
-              });
+              }).overrideScope' packageOverrides;
 
               allExtensionFunctions = prevExtensionFunctions ++ [ extensions ];
               enabledExtensions =