Make tests play nice with the changes

Not mergeable: the tests fail for now. I suspect the problem comes from
the tests for `testCorrectFile`, where the function `checkFromLine` tries
to look up a wrong line number. Some more investigation would be needed
to figure out exactly what happens (probably should show the TSV file
being tested).

Anyway, here are the changes in this commit:
- Encapsulation of the TSV module in an `Internal` module, so we can export
  just the interface, but still expose the internals to the tests
- Correction of type errors in the tests to have something that typechecks
  with the changes introduced before