I had no idea that `src_dirs` existed.  I'll remove the `rebar.config.script` hackery in favor of it, if I can get it to work.  Simply using:

    {src_dirs, ["./priv/tracers", "./src"]}.

... as the first line in `rebar.config` doesn't appear to behave.  :-(  I'll twiddle a bit more to try to understand what Rebar is (not) doing.