tree-wide: replace uses of splitString "." with lib.versions

Quoting from the splitString docstring:

   NOTE: this function is not performant and should never be used.

This replaces trivial uses of splitString for splitting version
strings with the (potentially builtin) splitVersion.

pkgs/development/libraries/science/math/cudnn/generic.nix

@@ -46,7 +46,7 @@ stdenv.mkDerivation {
 
   passthru = {
     inherit cudatoolkit;
-    majorVersion = lib.head (lib.splitString "." version);
+    majorVersion = lib.versions.major version;
   };
 
   meta = with stdenv.lib; {