Print a warning after `using Foo: bar` if `bar` is not public API

Now that #50105 has been merged, can we print a warning after `using Foo: bar` if `bar` is not marked with either `export` or `public`? Is adding a warning where there was none before considered a breaking change?

Having a warning would address one of the concerns in #42080, which is that encouraging `using Foo: bar` is likely to lead to folks bringing internal names into scope.