This commit adds a new `split-debuginfo` option to Cargo compilation
profiles which gets forwarded to the `-Csplit-debuginfo` codegen option
in rustc. This commit also sets the default, only on macOS, to be
`-Csplit-debuginfo=unpacked`. The purpose of this change is to leverage
rust-lang/rust#79570 to avoid running `dsymutil` on incremental builds
while also preserving a pleasant debugging experience by default. This
should lead to much faster incremental build times on macOS since
`dsymutil` isn't exactly the speediest tool in the world.
This is technically a breaking change in Cargo because we're no longer
by-default producing the `*.dSYM` folders on macOS. If those are still
desired, however, authors can always run `dsymutil` themselves or
otherwise configure `split-debuginfo = 'packed'` in their
manifest/profile configuration.
Prior to this commit `cargo metadata` would not emit the value of a
target's "doc" setting, used by `cargo doc` to determine whether
documentation should be generated. However, this information is useful
for machine programs interested in such targets, and the information is
already made available on the internal representation of a target, so
this commit just exposes that during target serialization for emit.
cf https://github.com/deadlinks/cargo-deadlinks/issues/99
