Don't emit warning for explicitly included data files

[cdce8p]

Summary of changes

From what I can tell based on the discussion in #3340, it would make sense to hide the Package would be ignored warning for explicitly including data files (via tool.setuptools.package-data or similar).

[abravalheri]

I have a different understanding of the previous discussion: if the package is not included, then the data files contained by it cannot be included (adding the package itself is a pre-requisite for adding its data files)...

So the warning is a call to action for the users to add the package name in the package list (this would be along the lines of "explicit is better than implicit" and raising awareness of the users that for Python purposes the directories are packages during runtime and can even be imported).