Added tracing (logging) to crate_universe (#2224)

The `CARGO_BAZEL_DEBUG` environment variable will now activate verbose
logging for the `cargo_bazel` binary (aka, in `crates_repository` and
`crates_vendor` rules). An example of the output looks like the
following:
```
~/Code/repro-cargo-bazel-lock-issue (main ✗) CARGO_BAZEL_DEBUG=1 REPIN=1 bazel build ...
Splice 2023-10-29T22:18:42.240171Z DEBUG LockGenerator::generate: cargo_bazel::metadata: Generating Cargo Lockfile for /private/var/tmp/_bazel_user/0c9b227e7e0599ba0ec95785817f1b2a/external/crate_index/splicing-workspace/Cargo.toml
Splice 2023-10-29T22:18:42.240217Z DEBUG LockGenerator::generate: cargo_bazel::metadata: Using existing lock /Users/user/Code/repro-cargo-bazel-lock-issue/Cargo.lock
Splice 2023-10-29T22:18:44.424638Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: Generating features for manifest /private/var/tmp/_bazel_user/0c9b227e7e0599ba0ec95785817f1b2a/external/crate_index/splicing-workspace/Cargo.toml
Splice 2023-10-29T22:18:44.424645Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: Spawning processes for {"aarch64-apple-darwin", "aarch64-apple-ios", "aarch64-apple-ios-sim", "aarch64-fuchsia", "aarch64-linux-android", "aarch64-pc-windows-msvc", "aarch64-unknown-linux-gnu", "arm-unknown-linux-gnueabi", "armv7-linux-androideabi", "armv7-unknown-linux-gnueabi", "i686-apple-darwin", "i686-linux-android", "i686-pc-windows-msvc", "i686-unknown-freebsd", "i686-unknown-linux-gnu", "powerpc-unknown-linux-gnu", "riscv32imc-unknown-none-elf", "riscv64gc-unknown-none-elf", "s390x-unknown-linux-gnu", "thumbv7em-none-eabi", "thumbv8m.main-none-eabi", "wasm32-unknown-unknown", "wasm32-wasi", "x86_64-apple-darwin", "x86_64-apple-ios", "x86_64-fuchsia", "x86_64-linux-android", "x86_64-pc-windows-msvc", "x86_64-unknown-freebsd", "x86_64-unknown-linux-gnu", "x86_64-unknown-none"}
Splice 2023-10-29T22:18:44.734294Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: Process complete for aarch64-apple-darwin
Splice 2023-10-29T22:18:44.734398Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: 	For anyhow 1.0.75 features were: {"default", "std"}
Splice 2023-10-29T22:18:44.734424Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: 	For autocfg 1.1.0 features were: {}
Splice 2023-10-29T22:18:44.734426Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: 	For base64 0.21.5 features were: {"alloc", "default", "std"}
Splice 2023-10-29T22:18:44.734430Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: 	For bitflags 2.4.1 features were: {"std"}
Splice 2023-10-29T22:18:44.734431Z DEBUG FeatureGenerator::generate: cargo_bazel::metadata: 	For bytes 1.5.0 features were: {"default", "std"}
```