Refactor Cask::Container, explicit vs autodetected container types

Refactor `Cask::Container`
- ttf, otf, pkg all get explicit types instead of being lumped under `Cask::Container::Naked`
- ttf, otf, pkg also get content tests as with other types
- keep `Cask::Container::Naked`, but don't include it in autodetected types
- document that `:naked` and `:generic_unar` are available as explicit container types (via stanza `container_type`) though they will not be autodetected
- cleanups
- test suite improvement