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/applications/misc/mupdf/default.nix

@@ -9,8 +9,7 @@ let
 
   # OpenJPEG version is hardcoded in package source
   openJpegVersion = with stdenv;
-    lib.concatStringsSep "." (lib.lists.take 2
-      (lib.splitString "." (lib.getVersion openjpeg)));
+    lib.versions.majorMinor (lib.getVersion openjpeg);
 
 
 in stdenv.mkDerivation rec {