You can enable Checkstyle's JavadocPackage check. Leave the allowLegacy
property at its default value of false
in order to ensure that people use a package-info.java instead of package.html.
There is no documented way that I know of to change the FindBugs defaults for null analysis annotations. So your next task may be to make sure that every package-info.java contains the appropriate annotation ...
Let me add a piece of un-asked-for advice: Personally, I would advise against using the defaults annotations, and instead explicitly annotate every method argument that must be non-null. This means that the default will be nullable, but FindBugs is clever enough to check the method code for parts which assume an argument to be non-null and flag that as an error. For large code bases, this is more reliable and easier from the governance point of view. Of course this path may be unavailable to you if you've got an existing code base.