file_packager files if source pathname contains @ sign

Our CI system generates pathnames with @ in them. But, the file_packager uses @ in the `--preload` command line arg. The file_packager is not able to handle this case. E.g, we end up with args like:
`--preload {root_path}@{folder_path}Required@/data/Required`

My suggested fix is to change this line of code to use `rfind` instead of `find`. I have tested and verified this fix in our CI system.

https://github.com/emscripten-core/emscripten/blob/1a623ed165713a45a113e37bd1d7d67b883c6a16/tools/file_packager.py#L240