fix(compiler): allow to run directly from bin directory

[michallepicki]

Before:

~/.opam/default/bin$ cat main.ml
let main _ = Io.format "~s~n" ["Hello World"]
~/.opam/default/bin$ ./caramel compile main.ml 
Unbound module Beam

After:

~/.opam/default/bin$ ./caramel compile main.ml 
Compiling main.erl      OK