This reduces the surface area of our `Kernel` monkeypatch and removes
the need to `include Kernel` in a bunch of modules.
While we're here, also move `Kernel#require?` to `Homebrew` and fully
scope the calls to it.
Taps can include completion scripts for external commands under
`completions/bash`, `completions/fish`, or `completions/zsh`. `brew tap`
will automatically install these into the correct directories during
install.
