Add windows-targets crate to std's sysroot
With this PR, when backtrace is used as a crate from crates.io it will (once updated) use the real [windows-targets](https://crates.io/crates/windows-targets) crate. But when used from std it'll use std's replacement version.
This allows sharing our customized `windows_tagets::link!` macro between std proper and the backtrace crate when used as part of std, ensuring a consistent linking story. This will be especially important once we move to using [`raw-dylib`](https://doc.rust-lang.org/reference/items/external-blocks.html#dylib-versus-raw-dylib) by default.
This ensures that the Cargo.lock packaged for it in the rust-src
component is up-to-date, allowing rust-analyzer to run cargo metadata on
the standard library even when the rust-src component is stored in a
read-only location as is necessary for loading crates.io dependencies of
the standard library.
This also simplifies tidy's license check for runtime dependencies as it
can now look at all entries in library/Cargo.lock without having to
filter for just the dependencies of runtime crates. In addition this
allows removing an exception in check_runtime_license_exceptions that
was necessary due to the compiler enabling a feature on the object crate
which pulls in a dependency not allowed for the standard library.
While cargo workspaces normally enable dependencies of multiple targets
to be reused, for the standard library we do not want this reusing to
prevent conflicts between dependencies of the sysroot and of tools that
are built using this sysroot. For this reason we already use an unstable
cargo feature to ensure that any dependencies which would otherwise be
shared get a different -Cmetadata argument as well as using separate
build dirs.
This doesn't change the situation around vendoring. We already have
several cargo workspaces that need to be vendored. Adding another one
doesn't change much.
There are also no cargo profiles that are shared between the root
workspace and the library workspace anyway, so it doesn't add any extra
work when changing cargo profiles.
