The install script at the root folder installs the tools used throughout the rest of the process and then sources the install script of each configuration module (Zsh, Vim, Git, etc).
A configuration module install script should rely on tools installed by the root install script (namely Homebrew and Homebrew Cask) and tools available in a default Mac OS X - El Capitan installation. There should be no dependencies on the previous execution of other configuration modules or manually installed tools.