Tests from non-exposed modules can be tested if they're reexported from an exposed module

[martijnbastiaan]

As brought up in #6, doctest-parallel currently errors if it finds a doctest that is part of an internal module. This is usually the right thing to do: it couldn't access the function under test anyway. However, if that particular function is re-exported from an exposed module, it could technically access it. My guess is that this is currently hard-ish to implement, however, it should be easy once https://gitlab.haskell.org/ghc/ghc/-/merge_requests/6224 is merged as we could easily compare names.

For me personally this is not a particularly high-priority issue (hence the enhancement label):

• doctest-parallel currently errors in these cases, no tests will be silently ignored.
• In almost all cases I consider hiding modules (and symbols) an anti-feature. If users want to use an internal symbol we (library authors) should let them. I'm therefore skeptical of the use of non-exposed modules in the first place.