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/interpreters/lfe/generic-builder.nix

@@ -9,9 +9,9 @@
 }:
 
 let
-  inherit (stdenv.lib) getVersion versionAtLeast splitString head;
+  inherit (stdenv.lib) getVersion versionAtLeast versions;
 
-  mainVersion = head (splitString "." (getVersion erlang));
+  mainVersion = versions.major (getVersion erlang);
 
   proper = buildHex {
     name = "proper";