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/compilers/cudatoolkit/default.nix

@@ -180,9 +180,7 @@ let
       '';
       passthru = {
         cc = gcc;
-        majorVersion =
-          let versionParts = lib.splitString "." version;
-          in "${lib.elemAt versionParts 0}.${lib.elemAt versionParts 1}";
+        majorVersion = lib.versions.majorMinor version;
       };
 
       meta = with stdenv.lib; {