An `@_export public import` is meaningful on it's own as it declares a
relationship between two modules and how clients see them. As such that
import doesn't have to be referenced from API to be appropriate. Let's
not warn on any public import with an `@_export` attribute.
rdar://122032960
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.
