FormulaInstaller: fix "optional build-time dep" edge case
FormulaInstaller: fix "optional build-time dep" edge case

An "optional build-time dep" is unlikely, but possible, and I happen to
notice this wasn't handled correctly while digging around in the deps
code over the last few days.

DomT4/brew,DomT4/brew,ilovezfs/brew,amar-laksh/brew_sudo,Linuxbrew/brew,aw1621107/brew,mahori/brew,gordonmcshane/homebrew,mistydemeo/homebrew,AnastasiaSulyagina/brew,EricFromCanada/brew,jmsundar/brew,reelsense/brew,Homebrew/brew,bfontaine/brew,jmsundar/brew,nandub/brew,toonetown/homebrew,staticfloat/brew,vitorgalvao/brew,EricFromCanada/brew,claui/brew,maxim-belkin/brew,rwhogg/brew,mahori/brew,reelsense/brew,muellermartin/dist,ilovezfs/brew,reelsense/brew,MikeMcQuaid/brew,AnastasiaSulyagina/brew,hanxue/linuxbrew,mahori/brew,tonyg/homebrew,sjackman/homebrew,mistydemeo/homebrew,gregory-nisbet/brew,gregory-nisbet/brew,Linuxbrew/brew,Linuxbrew/brew,jmsundar/brew,pseudocody/brew,Homebrew/brew,palxex/brew,AnastasiaSulyagina/brew,maxim-belkin/brew,nandub/brew,DomT4/brew,JCount/brew,reitermarkus/brew,JCount/brew,staticfloat/brew,alyssais/brew,claui/brew,EricFromCanada/brew,sjackman/homebrew,zmwangx/brew,maxim-belkin/brew,bfontaine/brew,konqui/brew,amar-laksh/brew_sudo,vitorgalvao/brew,Homebrew/brew,aw1621107/brew,mahori/brew,gordonmcshane/homebrew,JCount/brew,vitorgalvao/brew,hanxue/linuxbrew,muellermartin/dist,mgrimes/brew,amar-laksh/brew_sudo,ilovezfs/brew,mgrimes/brew,vitorgalvao/brew,rwhogg/brew,bfontaine/brew,Linuxbrew/brew,claui/brew,alyssais/brew,palxex/brew,hanxue/linuxbrew,MikeMcQuaid/brew,DomT4/brew,palxex/brew,rwhogg/brew,reitermarkus/brew,Homebrew/brew,nandub/brew,MikeMcQuaid/brew,tonyg/homebrew,claui/brew,toonetown/homebrew,pseudocody/brew,muellermartin/dist,toonetown/homebrew,konqui/brew,zmwangx/brew,gregory-nisbet/brew,JCount/brew,konqui/brew,zmwangx/brew,tonyg/homebrew,sjackman/homebrew,reitermarkus/brew,konqui/brew,staticfloat/brew,pseudocody/brew,alyssais/brew,nandub/brew,aw1621107/brew,gordonmcshane/homebrew,EricFromCanada/brew,MikeMcQuaid/brew,reitermarkus/brew,mistydemeo/homebrew,mgrimes/brew,sjackman/homebrew