@include_once libphutil, not @require_once
@include_once libphutil, not @require_once

(Committing on behalf of @cpiro)

Summary: there's a check beneath to see if it worked, but @require_once will
fail silently if libphutil isn't available

Test Plan: tried it with libphutil findable and not findable, got expected
behavior

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1018

folsom-labs/arcanist,aik099/arcanist,zhihu/arcanist,dguo-coursera/arcanist,phacility/arcanist,zhihu/arcanist,jra3/arcanist,folsom-labs/arcanist,memsql/arcanist,KieranHayes/arcanist,wxstars/arcanist,KieranHayes/arcanist,sneakpeeq/arcanist,phacility/arcanist,zhihu/arcanist,calblueprint/arcanist,zhihu/arcanist,Symplicity/arcanist,Khan/arcanist,alexmv/arcanist,julkiewicz/arcanist,sneakpeeq/arcanist,julkiewicz/arcanist,eSpark/arcanist,codelabs-ch/arcanist,r4nt/arcanist,PageUpPeopleOrg/arcanist-patches,coursera/arcanist,memsql/arcanist,kevinseelbach/arcanist,julkiewicz/arcanist,eSpark/arcanist,r4nt/arcanist,chundongwang/arcanist,uber/arcanist,codelabs-ch/arcanist,wxstars/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,KieranHayes/arcanist,kevinseelbach/arcanist,wikimedia/phabricator-arcanist,coursera/arcanist,jra3/arcanist,haskell-infra/arcanist,wharris/arcanist,aik099/arcanist,memsql/arcanist,aik099/arcanist,sneakpeeq/arcanist,hach-que/arcanist,alexmv/arcanist,Symplicity/arcanist,wikimedia/phabricator-arcanist,fivestars/arcanist,eSpark/arcanist,dguo-coursera/arcanist,wharris/arcanist,folsom-labs/arcanist,Khan/arcanist,aik099/arcanist,julkiewicz/arcanist,PageUpPeopleOrg/arcanist-patches,Khan/arcanist,chundongwang/arcanist,wharris/arcanist,Symplicity/arcanist,coursera/arcanist,phacility/arcanist,calblueprint/arcanist,codelabs-ch/arcanist,calblueprint/arcanist,hach-que/arcanist,phacility/arcanist,r4nt/arcanist,alexmv/arcanist,calblueprint/arcanist,wxstars/arcanist,codelabs-ch/arcanist,Symplicity/arcanist,wikimedia/phabricator-arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,fivestars/arcanist,kevinseelbach/arcanist,haskell-infra/arcanist,r4nt/arcanist,fivestars/arcanist,chundongwang/arcanist,wxstars/arcanist,phacility/arcanist,sneakpeeq/arcanist,alexmv/arcanist,jra3/arcanist,chundongwang/arcanist,dguo-coursera/arcanist,Khan/arcanist,fivestars/arcanist,wharris/arcanist,KieranHayes/arcanist,uber/arcanist,eSpark/arcanist,dguo-coursera/arcanist,hach-que/arcanist,uber/arcanist,hach-que/arcanist,codelabs-ch/arcanist,uber/arcanist,haskell-infra/arcanist,folsom-labs/arcanist,memsql/arcanist,kevinseelbach/arcanist,PageUpPeopleOrg/arcanist-patches,coursera/arcanist