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.

nixos/modules/services/web-servers/apache-httpd/default.nix

@@ -12,7 +12,7 @@ let
 
   php = mainCfg.phpPackage.override { apacheHttpd = httpd.dev; /* otherwise it only gets .out */ };
 
-  phpMajorVersion = head (splitString "." php.version);
+  phpMajorVersion = lib.versions.major (lib.getVersion php);
 
   mod_perl = pkgs.apacheHttpdPackages.mod_perl.override { apacheHttpd = httpd; };