file-packager js output not working with closure and --pre-js

I'm getting the following closure error when adding the output of file-packager using `--pre-js`:

> ERROR - [JSC_VAR_MULTIPLY_DECLARED_ERROR] Variable Module declared more than once.

Using `--preload-file` instead works fine which leads me to believe this is a bug.

Commands for completeness:
```bash
file_packager.py testapp.resources.data --preload testapp/resources@resources --js-output=testapp.resources.js --use-preload-cache
em++ -s WASM=1 -DNDEBUG -O3 -s ENVIRONMENT=web -s MODULARIZE -s EXPORT_NAME=createModule -flto --closure 1 -s FORCE_FILESYSTEM --pre-js testapp.resources.js [...]
```

Run the supplied file_packager.py and em++ commands with Closure enabled, then inspect testapp.resources.js for repeated Module declarations and compare the --preload-file path. Done means the generated file-packager output can be passed via --pre-js with --closure 1 without the reported JSC_VAR_MULTIPLY_DECLARED_ERROR.

Scritto dal modello di indicizzazione a partire dal testo della issue.