Remove `@_implementationOnly` annotations

[keith]

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

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

Mirrors https://github.com/apple/swift-syntax/pull/2429

Checklist

• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines

[keith]

@swift-ci please test

[keith]

@swift-ci please test

[keith]

@natecook1000 can you review? (sorry I can't add you as a reviewer here)

[rauhul]

Is there a reason to use private instead of internal?

Additionally could we use an upcoming feature flag instead of conditional compilation?

[keith]

I don't have a pref on private vs internal for this case, was just trying to mirror similarly to what was here before. But I guess if private works I'd think we should prefer that, wdyt? Based on https://github.com/apple/swift-syntax/pull/2429#discussion_r1462534192 you cannot use that flag instead

[rauhul]

I wasn't sure if there was a specific reason to prefer private other than minimally scoped imports. Given the context of the swift-syntax comment, this changes lgtm, but I'll wait for Nate to do the final approval

[keith]

@swift-ci please test