Never skip modules and packages

[sachahu1]

This fixes #448 and #441 which happen because missing option "undoc-members" causes modules and packages not to be documented. This enforces that all modules and all packages will always be documented.

[AWhetter]

Please can you add test(s) for this.

[sachahu1]

@AWhetter Added some simple integration tests

[sachahu1]

@AWhetter Are you happy to merge this?

[AWhetter]

As mentioned here, I think that the current behaviour is correct. What I failed to do was introduce this change in a major version.
If there's a need to be able to configure the rendering of undocumented members separately from the rendering of undocumented modules then we could consider adding an undoc-modules option to autoapi_options. But the list of options is already quite long so I'd want to see multiple users asking for it, to justify the extra complexity. We'd also need to introduce this change in a major version.
Thank you for going above and beyond to bring this issue to my attention.