Fixes adding multiple files to versioned packages

This fixes https://trello.com/c/9yegUm4V/233-bug-when-editing-a-versioned-item-data-files-are-duplicated.

To test: 
* create a package, take it all the way to archiving. (10.5061/dryad.xxxxx, 10.5061/dryad.xxxxx/1)
* make a new version of the package. (10.5061/dryad.xxxxx.2, 10.5061/dryad.xxxxx.2/1.2)
* when editing the new version, add more than one data file. (10.5061/dryad.xxxxx.2/2.2, 10.5061/dryad.xxxxx.2/3.2)
You should be able to add more than one data file correctly now. 

In addition, when you try to access the versioned files, you should only be able to find the versioned files in the versions of the packages they're in, but not in previous versions:
10.5061/dryad.xxxxx/3 points to 10.5061/dryad.xxxxx.2/3.2 (but 10.5061/dryad.xxxxx.1/3.1 does not exist)