Remove @_implementationOnly annotations

[marcosgriselli]

These annotations produce warnings when compiling without library evolution using Swift ≥5.10.

Replace them by private import when compiling using Swift ≥5.11.

Mirrors swiftlang/swift-syntax#2429

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[marcosgriselli]

@natecook1000 sorry for the ping, can we add a reviewer for this?

[LucianoPAlmeida]

@swift-ci Please test

[natecook1000]

Looks great, thanks @marcosgriselli!

[tshortli]

Importing RealModule without @_implementationOnly in Swift 5.10, but with @_implementationOnly in Swift 5.9 doesn't make sense to me. With @_implementationOnly the dependency is hidden from dependents entirely. Without it, the dependency is exposed to dependents. If in practice it is ok to expose the RealModule dependency to dependents (as it seems it must be) then it would be more consistent to simply remove @_implementationOnly for <= 5.10, rather than having different semantics for 5.10 and <= 5.9.