binary_deps requires a root package to exists

[UebelAndre]

The issue

When using cargo workspaces where it's possible to not have a root package will run into the following issue when using the binary_deps setting

Error: Failed to fetch Metadata with `/private/var/tmp/_bazel_user/09e1dd6c594fb76d12b4c7a0985301d8/execroot/repo/bazel-out/darwin-fastbuild/bin/external/cargo_raze/tools/raze.runfiles/repo/external/rust_darwin_x86_64/bin/cargo` from `/var/folders/hf/phjl9q7501gb5wt_qr4ltn3m0000gn/T/.tmpAgj6wx`

Caused by:
    `cargo metadata` exited with an error: error: failed to parse manifest at `/private/var/folders/hf/phjl9q7501gb5wt_qr4ltn3m0000gn/T/.tmpAgj6wx/Cargo.toml`

    Caused by:
      this virtual manifest specifies a [[bin]] section, which is not allowed

This is because when using binary_deps, the cargo manifest tree is symlinked into a new directory and a modified root Cargo.toml is generated in an incomplete state.

The fix

To avoid this issue, simply define a fake root package.

# This package is only here to satisfy bad behavior in `cargo-raze`. Where
# the modified root `Cargo.toml` file requires a package when using `binary_deps`.
# This the tracking issue is: https://github.com/google/cargo-raze/issues/404
# 
[package]
name = "root_package"
version = "0.0.0"
[lib]
name = "root_package_cargo_raze_only"
path = ".cargo_raze.rs"

[UebelAndre]

Hopefully this field and all it's code can be removed: #305