Validate `cargo package` contents in CI

Add a CI test that runs `cargo package` or `cargo publish --dry-run` or similar, and validates certain aspects of the resulting artifacts. This should include:
- Validate that the `LICENSE` file was included, and that its contents are correct
- This is especially important since zerocopy-derive's license file was spuriously removed in https://github.com/google/zerocopy/pull/275 and re-added later in https://github.com/google/zerocopy/pull/489

This issue requires both identifying what contents we should be checking for and then writing the test. It's okay if this work is done incrementally (e.g., first checking for a single file, then adding more checks over time).