I don't think attoparsec or any of its dependencies have any native requirements, and some poking around seems to support this, although I don't have any easy way of telling for certain (which is part of what I'm looking for).
On the other hand, something like postgresql-libpq clearly depends on the libpq C library (and possibly other things) as listed in the Extra-Libraries field in its cabal file, *as do anything that require postgresql-libpq* which fact does *not* directly show up in their cabal file. Walking the tree and pulling that info is a possibility, but it needs to be done after dependency-resolution in case requirements change version to version.