[Normalizer] imported_names in Env instead of State
[Normalizer] imported_names in Env instead of State

Summary:
This is another portion that will not change during normalization so does
not need to be inside State. This allows up to pull `symbol` outside the monad.

Reviewed By: nmote

Differential Revision: D8238754

fbshipit-source-id: c9b664eb55e5813c91873268c30bc57974d1c4bb

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