swap: create .wants symlink to 'auto' swap devices

As we load unit files lazily, we need to make sure something pulls in swap
units that should be started automatically, otherwise the default dependencies
will never be applied.

This partially reinstates code removed in
commit 64347fc2b983f33e7efb0fd2bb44e133fb9f30f4.

Also don't order swap devices after swap.target when they are 'nofail'.