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.
- move some things out of `extend` that don't really fit there e.g.
`Module`s that are included but not doing any
overriding/monkeypatching
- move some code into `extend/os` to fix all remaining
`rubocop:todo Homebrew/MoveToExtendOS`s
- remove some unneeded `bundle` skipper code that doesn't really make
sense given our current bottling strategy
- extract some `Pathname` extensions to `extend/pathname` for separate
files
- move a `ENV` `Kernel` extension into `kernel.rb`
- `odeprecate` a seemingly unused backwards compatibility method
- move `readline_nonblock` from a monkeypatch to a
`ReadlineNonblock.read` method as its only used in one place
- fix up a link in documentation
