Don't subdirectory beams.

In the case of a beam called Coq.Arith.EqNat, which will be called
directly in the Core Erlang like call 'Coq.Arith.EqNat':'beq_nat', don't
put into a subdirectory or the emulator will not be able to find it.

src/rebar_erlc_compiler.erl

@@ -377,7 +377,7 @@ internal_core_compile(Config, Source, Outdir, ErlOpts) ->
     {Module, Hrls} = inspect(Source, include_path(Source, Config)),
 
     %% Construct the target filename
-    Target = filename:join([Outdir | string:tokens(Module, ".")]) ++ ".beam",
+    Target = filename:join([Outdir,Module]) ++ ".beam",
     ok = filelib:ensure_dir(Target),
 
     %% If the file needs compilation, based on last mod date of includes or