tighten file sig for namespace exports
tighten file sig for namespace exports

Summary: A list that is either empty or a singleton can be represented as an option.

Reviewed By: samwgoldman

Differential Revision: D9143172

fbshipit-source-id: 1597013dc749d8f926b56f648767fb2ed709f609

JonathanUsername/flow,popham/flow,JonathanUsername/flow,facebook/flow,AgentME/flow,AgentME/flow,jamesgpearce/flow,facebook/flow,JonathanUsername/flow,gabelevi/flow,JonathanUsername/flow,nmote/flow,mroch/flow,nmote/flow,popham/flow,nmote/flow,AgentME/flow,mroch/flow,jamesgpearce/flow,nmote/flow,nmote/flow,facebook/flow,popham/flow,JonathanUsername/flow,samwgoldman/flow,facebook/flow,jamesgpearce/flow,nmote/flow,AgentME/flow,gabelevi/flow,samwgoldman/flow,JonathanUsername/flow,samwgoldman/flow,jamesgpearce/flow,nmote/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,jamesgpearce/flow,mroch/flow,jamesgpearce/flow,facebook/flow,mroch/flow,gabelevi/flow,samwgoldman/flow,mroch/flow,popham/flow,JonathanUsername/flow,jamesgpearce/flow,popham/flow,mroch/flow,popham/flow,popham/flow,samwgoldman/flow,gabelevi/flow,AgentME/flow,mroch/flow,facebook/flow,samwgoldman/flow,gabelevi/flow,gabelevi/flow