Fix CARGO_MANIFEST_DIR when transform_sources moves crate root to bazel-out

[0xbeefd1ed]

Summary

When a rust_library (or similar rule) has a mix of generated and non-generated files in compile_data, transform_sources symlinks all source files into bazel-out/. However, CARGO_MANIFEST_DIR was still computed from ctx.label.workspace_root and ctx.label.package, pointing to the original source directory. This caused proc-macros that read Cargo.toml (e.g., Bevy's #[derive(Component)]) to fail with "Cargo manifest does not exist" errors in sandboxed builds on Linux / macOS.

This change derives CARGO_MANIFEST_DIR from the actual crate root location when the crate root has been transformed, using crate_info.root.root.path combined with the package path.

Reproduction

1. Create a rust_library with a generated file in compile_data and Cargo.toml also in compile_data
2. Use a proc-macro that reads Cargo.toml via CARGO_MANIFEST_DIR (e.g., Bevy's derive macros)
3. Build with sandboxing enabled (default on Linux)
4. Observe: proc-macro panics because Cargo.toml is not found at the CARGO_MANIFEST_DIR path

Fix

• If crate_info.root.is_source, use the existing label-based path (no behavior change)
• Otherwise, compute the manifest directory from crate_info.root.root.path + the workspace/package path, which reflects the actual bazel-out/ layout

Disclosure

This PR was created with assistance from Claude Code.

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