[Tests] Update swiftinterface imports Swift 6 test
Update the test for imports defaulting to internal by adding the
explicit `public` modifier on all imports and the expected warnings.
Modernized a test somehow as it can't use previously shared files.
This revealed an issue with the warning about superfluous imports on
imports of submodules. I've marked one of them as a FIXME as it will
require a fix in the diagnostic.