test/Doctests.hs: support new Cabal autogen path in doctests.

Since the last haeredes release, Cabal has moved its auto-generated
modules from dist/build/autogen to dist/build/$component/autogen,
breaking our doctests which need to load the Paths_haeredes module.
These paths are passed to the doctest runner as a list, so it's easy
to support both: we now just pass both paths.