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.
- Homebrew Bundle referred to formulae as "brews". But it referred to
casks as "casks" and taps as "taps".
- Let's use the same terminology everywhere.
- (I know that `brew "hello"` is the formula syntax in the Brewfile, so
I'm not changing that (though would be up for it, in a backwards
compatible manner), just making the code more consistent.)