Refactor and document `PkgVersion`.
Refactor and document `PkgVersion`.

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