Declare rust_packages only when installing Rust IDL bindings

[mxgrey]

As discussed here there was a strange error that made it impossible to use ros2_rust in an underlay workspace. Packages that have no association to Rust were declaring themselves in the rust_packages ament index. This was causing downstream colcon workspaces to redirect their .cargo/config.toml file towards non-existent directories. When cargo is directed towards non-existent directories, it considers the situation a build error and exits immediately.

After some challenging investigation, I narrowed the problem down to this: Packages that contain any message definitions were invoking the rosidl_generator_rs's extension to the ROS IDL generation pipeline. That extension included the line ament_index_register_resource("rust_packages"). For a normal message package that would be fine, but some packages have message definitions that are purely internal to the package for testing purposes and not installed (using the SKIP_INSTALL option of rosidl_generate_interfaces). Since the messages are not installed in these cases, we also do not install the Rust bindings, which makes perfect sense. However, if we declare the package under the rust_packages index, then colcon-ros-cargo will expect a Rust package even though one does not exist.

This PR solves the problem in an extremely simple way: The rosidl_generator_rs extension will only declare a package into the rust_packages index if the message package is being installed. I've done an A/B test on this change and confirmed that building an rclrs node in a downstream workspace is impossible without this, but works perfectly fine with the changes in this PR.

[esteve]

@mxgrey thanks!