qt5.qtwebkit: Fixes modules src being subtly broken.

b785d4813e introduced breakage in Qt
modules for 5.6 and 5.9, especially visible is Qt Webkit.

This was manifested by having a non-sensical build log where it is using
the top-level `src` attribute as source instead of Qt Webkit's own
source.

Were it not for the `src` top-level attribute (which is a legit
package), the error would have been made obvious by passing `null` to
`src`.

This partily reverts newly introduced way `src` can be passed to a
qtModule, instead relying on extending the `srcs` attrset.

For ZHF #45960

pkgs/development/libraries/qt-5/qtModule.nix

@@ -8,7 +8,7 @@ args:
 
 let
   inherit (args) name;
-  version = if (args.version or null) == null then srcs."${name}".version else args.version;
+  version = args.version or srcs."${name}".version;
   src = args.src or srcs."${name}".src;
 in