Load formula classes into private namespace

Now that the formula file is no longer the executing script during builds, we can exert total control over the context in which formula classes are loaded, including the namespace in which they exist.

This is accomplished by reading the file and `class_eval`ing it inside a module: https://github.com/jacknagel/homebrew/blob/31546d1eae2733883d420f21134c919b5d6ff00e/Library/Homebrew/formulary.rb#L87

This means we will no longer conflict with standard library constants like `Readline` and `Zlib`.

:tada: