Support Payload-less Packages

[selfcommit]

This issue is being presented as an opportunity for the Github community to make any further suggestions about implementation before an attempt is made to implement a solution.

Work will start on or about July 13

Details from the Simian feature request doc are below:

Problem:
https://groups.google.com/forum/?fromgroups#!topic/simian-discuss/hT8IJjGtUmk
Not only does plist.PackageInfo enforce installer_item_(location|hash), but the admin UI also doesn't allow packages without associated DMGs to be placed into catalogs/manifests, etc.

Workaround:
Upload a dummy DMG.

Solution:
First, we need to make installer_item_* optional, and next we need to create a way for admins to opt to not upload a package DMG and still make the package deployable.