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.
Also, use `--no-same-permissions` instead of `-o` (which does nothing
without `-p` being passed and run as `root`) for the more explicit
description of the behaviour we want.
