Generate .cargo_vcs_info.json and include in `cargo package` (take 2)
Implements #5629 and supersedes #5786, with the following improvements:
* With an upstream git2-rs fix (tracked #5823, validated and min version update in: #5858), no longer requires windows/unix split tests.
* Per review in #5786, drops file system output and locks for .cargo_vcs_info.json.
* Now uses serde `json!` macro for json production with appropriate escaping.
* Now includes a test of the output json format.
Possible followup:
* Per discussion in #5786, we could improve reliability of both the VCS dirty check and the git head sha1 recording by preferring (and/or warning otherwise) the local repository bytes of each source file, at the same head commit. This makes the process more appropriately like an atomic snapshot, with no sentry file or other locking required. However given my lack of a window's license and dev setup, as exhibited by troubles of #5823, this feel intuitively like too much to attempt to change in one iteration here. I accept the "best effort" concept of this feature as suggested in #5786, and think it acceptable progress if merged in this form.
@alexcrichton r?
@joshtriplett cc
* Collapse the nested cargotest::support module into the cargotest
module (merge the mod.rs's)
* Rename the cargotest module to support
* Nest the top-level hamcrest module into support
