autodoc_mock_imports doesn't work as expected with new :recursive: option

[JamesALeedham]

Describe the bug
I would expect autodoc_mock_imports (and autosummary_mock_imports, the behavior seems to be the same) to exclude an entire module from a project that has been automatically iterated over using the :recursive: option available to sphinx.ext.autosummary since Sphinx 3.1.

It excludes the content of that module, but not the module itself, which ends up as an 'empty' entry in the summary table.

To Reproduce
Steps to reproduce:

1. Clone https://github.com/JamesALeedham/Sphinx-Autosummary-Recursion.
2. View the copy of the docs hosted here and observe that mytoolbox.mymodule2 is fully documented.
3. Edit source/conf.py and add the line autodoc_mock_imports = ['mytoolbox.mymodule2'].
4. Follow the instructions in the Github Readme to rebuild and view the docs locally.

Expected behavior
mytoolbox.mymodule2 is excluded from the summary table completely.

Actual behavior
mytoolbox.mymodule2 is still listed in the summary table. Click on it, though, and the module is empty - the class it contains is not documented.