Ignore paths tagged with -traverse while scanning

[disteph]

Minor point: Currently, Modules.scan_tree inspects all of the current directory and subdirectories, including my .git and my huge benchmark directory. Both are normally ignored by ocamlbuild because I put -traverse, not_hygienic tags. Namespaces happily goes there anyway. Might be worth looking up for the traverse tag before going in. Namespaces.delete_mllib_files should probably take that into account to.