Relay Compiler iterates over every file under root when Watchman is disabled

[sverrejoh]

Relay compiler falls back to using WalkDir to iterate over the filesystem when Watchman isn't available. The problem is that this implementation walks over every single file under root_dir, which can be your entire repository, rather than limiting it to the files being referenced by the Relay configuration.

This can be a lot of wasted work, and is going to be slow for larger mono-repositories.