Supporting package directory downloads

[guybedford]

I get upstream JSPM issues from time to time about the fact that JSPM modules reference relative paths like ../_/chunk-134234.js to load chunks for multi-entry packages that use chunking optimizations.

The workflows here don't support these by default by not downloading all files of the package into a location that matches the relative pathing.

I believe #235 and the associated work on a download listing API was the right direction to resolve this, but that work seemed to stall out.

Given it's a common issue and there isn't currently a tracking issue I wanted to post this for further follow-ups in future. Happy to discuss further or help where I can too.