Input root-relative output_paths

[Steven Bergsieker]

At the monthly sync, we discussed whether to update the remote apis spec to make output_paths relative to the input root rather than the working directory. This would be a breaking change for some implementations, so we're gathering input from the whole RE API community.

I have created https://github.com/bazelbuild/remote-apis/pull/191 based on the conversation at the monthly sync. Please leave comments on that PR.

Thanks,

Steven