The feature InternalImportsByDefault makes imports default to internal instead
of public. Applying the Swift 6 behavior of SE-0409 in Swift 5.
Let's use only that flag to track the Swift 6 behavior as well instead
of separately checking for the language version.
When using access level on imports, consider non-public imports to be
implementation details not exposed to clients. As such, a client loading
a library doesn't need to load non-public transitive dependencies.
This behave like `@_implementationOnly imports` at a module-wide level,
but it is restricted to resilient modules only. An import with any
access-level in a non-resilient module remains visible to transitive
clients.
