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 64347fc2b9.

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

src/core/swap.c

@@ -220,8 +220,12 @@ static int swap_add_default_dependencies(Swap *s) {
         }
 
         if (!noauto) {
-                r = unit_add_two_dependencies_by_name_inverse(UNIT(s), UNIT_AFTER, (nofail ? UNIT_WANTS : UNIT_REQUIRES),
-                                                      SPECIAL_SWAP_TARGET, NULL, true);
+                if (nofail)
+                        r = unit_add_dependency_by_name_inverse(UNIT(s),
+                                UNIT_WANTS, SPECIAL_SWAP_TARGET, NULL, true);
+                else
+                        r = unit_add_two_dependencies_by_name_inverse(UNIT(s),
+                                UNIT_AFTER, UNIT_REQUIRES, SPECIAL_SWAP_TARGET, NULL, true);
                 if (r < 0)
                         return r;
         }