keg: create subdirectories of lib/lua instead of symlinks

Some formulae (e.g. `luv`) have a `lib/lua/5.1` subdirectory inside
their keg. Before this change, the `5.1` subdirectory is symlinked into
`HOMEBREW_PREFIX`.

This can result in `luarocks` installing things into a formula's keg,
which we don't want.

Let's fix that by making sure that `brew link` creates these
subdirectories instead of symlinking them. We already do this for
subdirectories of `share/lua`:

8dd96ae8ba/Library/Homebrew/keg.rb (L430)

Library/Homebrew/keg.rb

@@ -446,7 +446,7 @@ class Keg
            /^gdk-pixbuf/,
            "ghc",
            /^gio/,
-           "lua",
+           /^lua/,
            /^mecab/,
            /^node/,
            /^ocaml/,