`--doctest-modules` option causes namespace packages' subpackages to be added to `sys.path`

Description

I created a simple namespace package with the following structure:

.
├── pyproject.toml
└── src
    ├── my_ns
    │   └── my_pkg
    │       ├── README.md
    │       └── __init__.py
    └── tests
        ├── __init__.py
        └── test_my_pkg.py

In the test_my_pkg.py file, I print the content of sys.path.

# test_my_pkg.py
from my_ns import my_pkg

import sys

def test_package_version():
    print("\n\nsys path = ", str(sys.path))
    assert my_pkg.__version__ == "0.0.1"

I noticed that when I execute pytest with the option --doctest-modules, src/my_ns is added to my sys.path.
While, without the option, src/my_ns it is not added (only src/, which makes sense).

I wanted to check with you if this is expected behaviour. In particular I have an external dependency called my_pkg. Which caused a weird conflict during pytest execution.

Env characteristics

pip list

Package        Version
-------------- -------
exceptiongroup 1.2.1
iniconfig      2.0.0
packaging      24.0
pip            23.0.1
pluggy         1.5.0
pytest         8.2.0
setuptools     65.5.0
tomli          2.0.1

pytest and operating system versions

pytest 8.2.0
macOS 14.4.1

• a detailed description of the bug or problem you are having
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

Start with the minimal namespace-package tree and run pytest with and without --doctest-modules, comparing sys.path in test_my_pkg.py. Trace the collection behavior for doctest modules and determine whether adding src/my_ns is intentional; done means the behavior is documented or corrected so the external my_pkg conflict is addressed.