Implicitly detect directories when updating a fragment from a ZIP

Summary: This fixes the update-from-ZIP functionality so that it will automatically detect directories in the ZIP that do not have explicit entries.  Some ZIP programs do not create directory entries explicitly, so if we fail to do this then there's no way for users to access the sub-fragments (even though they exist, there is no directory fragment to click through).

Test Plan: Created and updated fragments from a ZIP that had implicit directories in it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, staticshock

Maniphest Tasks: T4205

Differential Revision: https://secure.phabricator.com/D7733