It is possible that there may be two species which are so interdependent that one is necessary for the other to breed. (Zombie ants come to mind, but this would be on a different scale). I am interested in whether the relationship between facehuggers and the full xenomorph is a case of "facehugger is a part of the life-cycle of a xenomorph" or a case of "two inter-dependent species".
(By the nature of symbiosis, it is also possible that these two started as independent species and have combined into one super-organism.)
Almost certainly. The different stages' reproduction produces each other (facehuggers come from eggs, the other stages come from facehugger "infestation") so I think by any meaningful definition they'd have to be the same species.
The reason it doesn't look like it at first is that the facehugger dies rather than transforming into the next stage. This isn't just a normal larva to adult metamorphosis. (The chestburster to "alien" transition would be a metamorphosis though.)
Xenomorph starts life as an egg-like creature (first stage) which, when detecting the presence of a potential host, opens up to release the second stage: a small quasi-arachnoid life form (facehugger) which launches itself onto the host's face and impregnates it with an endoparasitoid larva, which later consumes the host from within and exits the body as a vaguely worm-like organism, which then grows very quickly, developing into the adult Xenomorph.
I would argue that is highly impropable that the different stages are different species living in symbiosis (By the way your example isn't - as the ant has no benefit from being infected by the fungus). If they were different species every stage would have to carry the genome of every other stage which would be highly complicated to achive. I can't say it would be impossible though.