keg: create subdirectories of `lib/lua` instead of symlinks
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`:

https://github.com/Homebrew/brew/blob/8dd96ae8bacb4f7eb3ef476af63470e3702aee35/Library/Homebrew/keg.rb#L430

Homebrew/brew,MikeMcQuaid/brew,nandub/brew,JCount/brew,nandub/brew,nandub/brew,JCount/brew,MikeMcQuaid/brew,JCount/brew,Homebrew/brew,JCount/brew,MikeMcQuaid/brew,nandub/brew,MikeMcQuaid/brew,Homebrew/brew,Homebrew/brew