Stop importing TimestampedData from package:analyzer/src.

[stereotype441]

As of analyzer version 8.1.0, the TimestampedData class is available in the analyzer public API at
package:analyzer/source/timestamped_data.dart.

Importing TimestampedData via the public API will allow analyzer internals to be cleaned up in the future.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.