[native] Stop using @_implementationOnly import in package configs

[lorentey]

@_implementationOnly does not work correctly unless Swift’s Library Evolution dialect is enabled — it leads to obscure module loading failures.

There seems to be no way to create a compile-time condition on whether library evolution is enabled, so we need to stop using @_implementationOnly imports in all package configurations.

Resolves #107.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [X] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

@swift-ci test