cargo's extra verbose mode is useful for getting detailed information out of
builds in CI where it can be difficult to examine the build environment
after-the-fact. However, when multiple build scripts are running as part of a
build it's not always clear what output is from which build script. This patch
makes cargo prefix each line of build script output with the crate name and
version this case.
- 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.
