Adding generated code in the default package is not supported as we
intend to import it, most probably from another package, and that is
not supported. While this situation is hard to replicate with Java,
Kotlin is unfortunately more lenient and users can end up in that
situation if they forget to add a package statement.
This commit checks for the presence of a valid package, and throws
a dedicated exception if necessary.
Closes gh-31628
f146d09607