link: link kegs without formulae.
link: link kegs without formulae.

Needed for `brew diy`.

Fixes #9036

sjackman/homebrew,claui/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,EricFromCanada/brew,sjackman/homebrew,Homebrew/brew,reitermarkus/brew,sjackman/homebrew,konqui/brew,claui/brew,reitermarkus/brew,konqui/brew,MikeMcQuaid/brew,JCount/brew,vitorgalvao/brew,EricFromCanada/brew,EricFromCanada/brew,nandub/brew,Homebrew/brew,reitermarkus/brew,JCount/brew,claui/brew,nandub/brew,Homebrew/brew,konqui/brew,claui/brew,sjackman/homebrew,konqui/brew,vitorgalvao/brew,MikeMcQuaid/brew,vitorgalvao/brew,JCount/brew,nandub/brew,nandub/brew,reitermarkus/brew,vitorgalvao/brew,EricFromCanada/brew,Homebrew/brew,JCount/brew