- Add newline at end of file.
- Remove unnecessary rustfmt changes.
- Move file writing code to `write_if_changed`.

- Use new enum `TargertSourcePath` for Target::src_path to make it explicit that metabuild has a special path.
- `cargo metadata` now skips the metabuild Target.
- JSON artifacts include the true path to the metabuild source file. This may not be the best solution, but it's unclear what it should be, and I would prefer to avoid breaking the output. Alternatively it could just not emit anything? I'm not completely familiar with the use case of these artifact messages.
- Place the file in `target/.metabuild/metabuild-pkgname-HASH.rs` instead of in the debug/release directory.  Its contents do not depend on the profile.
- Fix bug in write_if_changed.
- More tests.