Search inside ocamlbuild _build directory
By default ocamlspot tries to find an annotation file at the same directory of the corresponding source code. But there are build systems which do not follow this. For example, ocamlbuild.
Currently for those cases of "object files not in the source directory" are covered by placing .ocamlspot config files. We probably can introduce a special treatment for ocamlbuild, so that ocamlspot automatically searches _build directories even without .ocamlspot.