Linking to implicit namespace package with :py:mod: not working

[estan]

Describe the bug

The documentation for :py:mod: says that it can also be used to link to packages. This seems to work for regular packages, but not implicit namespace packages.

How to Reproduce

mkdir -p testcase/foo
cd testcase
touch foo/bar.py
python3 -m venv venv
source venv/bin/activate
pip install sphinx
mkdir source
echo 'import os; import sys; sys.path.insert(0, os.path.abspath("..")); extensions = ["sphinx.ext.autodoc"]' > source/conf.py
sphinx-apidoc -f --implicit-namespaces -o source foo
echo ':py:mod:`foo`' > source/index.rst
sphinx-build source build
xdg-open build/index.html

Notice how "foo" in the output is not a hyperlink.

On the other hand, if we turn foo into a regular package and re-run sphinx-apidoc / sphinx-build:

touch foo/__init__.py
sphinx-apidoc -f -o source foo
sphinx-build source build
xdg-open build/index.html

Then "foo" is now a hyperlink.

Expected behavior

I expect "foo" in the output to be a link to foo.html#foo-namespace.

Your project

See steps above

Screenshots

No response

OS

Ubuntu 20.04.3 LTS

Python version

3.8.10

Sphinx version

4.1.2

Sphinx extensions

sphinx.ext.autodoc

Extra tools

No response

Additional context

No response