Refactor VanguardHsPathBuilder::pick_path to be less complex

Currenty VanguardHsPathBuilder::pick_path contains some rather complicated logic for selecting the hops for HS paths (see the many different RelayExclusion rules). Moreover, the various RelayExclusion we appliy there are duplicated in hspool.rs, where we validate circuits and optionally extend them using the same logic as in VanguardHsPathBuilder::pick_path.

We should refactor it so that it's less fragile and easier to follow.