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
@_exported can only be used on public imports, it's incompatible with
any other access level. This implementation doesn't take into account
Swift 6 mode.
