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.
- make the description more generic/correct
- use "internet archive" over "archive"
- move some logic to a new `GitHubReleases` class (for consistency)
- remove some obvious comments
- extract out and move some constants
