XCodeDependency: Clarify that Xcode.app is needed
XCodeDependency: Clarify that Xcode.app is needed

Clarify the error message arising from XCodeDependency so that users know
XCode.app is needed and that just installing the CLT does not count as
"installing XCode".

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