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.
Some of the typing/`T.must` usage when moving the
Homebrew/homebrew-services code to Homebrew/brew was not quite correct.
Rather than trying to make everything `strict` and import at the same
time: let's mostly loosen the typing to fix a few bugs and allow us to
add more later.
