Fixes aspect-build#670.

As described in aspect-build#670, protoc splits the arguments to `--proto_path` and `--descriptor_set_in` using an OS-specific path-separator. On posix, this is `:`, but on Windows this is `;`. The protobuf library takes the approach for its bazel rules to join on `ctx.configuration.host_path_separator`, so I've taken the same approach here as well.

This only simply tests a basic message that depends on a message in another package, but for the scope of the current issue is appropriate. It's probably worth testing grpc functionality, too, but the examples package provides coverage.