Use warn(unused_crate_dependencies) in compiler crates.

[nnethercote]

The commit also removes fifteen unnecessary dependencies from Cargo.toml files that this finds. And tweaks some conditional dependencies.

There are three crates with false positives, relating to tests and doc comments. For these three crates allow(unused_crate_dependencies) is used along with an explanatory comment.

r? @ghost

[rust-log-analyzer]

The job x86_64-gnu-llvm-18 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

#19 exporting to docker image format
#19 sending tarball 20.1s done
#19 DONE 26.4s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Listening on address 127.0.0.1:4226
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10

[compiler-errors]

Perhaps investigate whether it's possible to globally enable the warnign for all compiler crates in somewhere like here: https://github.com/rust-lang/rust/blob/master/src/bootstrap/src/core/builder/cargo.rs#L1074

Then we can allow it in just the crates that need to be special, and we get it for free in new crates that get introduced ot the compiler.

[nnethercote]

Perhaps investigate whether it's possible to globally enable the warnign for all compiler crates

I've done this in #137930, I think it worked out pretty well.

[nnethercote]

I'll end up doing this in some kind of fashion where the unused_crate_dependencies can be added in a single spot. A mixture of #137930 and #138084.