main: fix only one doctest collected on pytest --doctest-modules __init__.py

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>.

changelog/8016.bugfix.rst

@@ -0,0 +1 @@
+Fixed only one doctest being collected when using ``pytest --doctest-modules path/to/an/__init__.py``.