tap_constants: allow formulae to have @ in name.
tap_constants: allow formulae to have @ in name.

Follow-up from #812 to fix handling fully-qualified versioned formulae
names.

Allows pulling https://github.com/Homebrew/homebrew-core/pull/971.

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