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/mesa/default.nix

@@ -28,7 +28,7 @@ with stdenv.lib;
 
 let
   version = "19.1.5";
-  branch  = head (splitString "." version);
+  branch  = versions.major version;
 in
 
 stdenv.mkDerivation {