When --doctest-modules is used, an `__init__.py` file is not a `Package`
but a `DoctestModule`, but some collection code assumed that
`__init__.py` implies a `Package`. That code caused only a single test
to be collected in the scenario in the subject.
Tighten up this check to explicitly check for `Package`. There are
better solutions, but for another time.
Report & test by Nick Gates <nickgatzgates@gmail.com>.
265cc2cfec