Fix package set serialization bugs (#1256)
Fix package set serialization bugs (#1256)

* Fixed incorrect variable usage
* Fixed big where pathlib.Path() cannot handle NoneType
* Cleanup process pool after work is done

quiltdata/quilt,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt-compiler