The public API should only access zeroconf and zeroconf.aio
as internals may be relocated between releases

Import them without the underscore in the public api. H11 is a good example of this at https://github.com/python-hyper/h11/blob/master/h11/__init__.py

Document that only zeroconf and zeroconf.aio are the public api and everything else may change

• .dns -> ._dns Mark zeroconf.dns as protected by renaming to zeroconf._dns #574
• .core -> ._core Mark zeroconf.core as protected by renaming to zeroconf._core #575
• .handlers -> ._handlers Mark zeroconf.handlers as protected by renaming to zeroconf._handlers #577
• .logger -> ._logger Mark zeroconf.logger as protected by renaming to zeroconf._logger #578
• .exceptions -> ._exceptions Mark zeroconf.exceptions as protected by renaming to zeroconf._exceptions #580
• .cache -> ._cache Mark zeroconf.cache as protected by renaming to zeroconf._cache #581
  .const -> ._const This one is already all protected
• .utils -> ._utils Mark zeroconf.utils as protected by renaming to zeroconf._utils #582
• .services -> ._services Mark zeroconf.services as protected by renaming to zeroconf._services #583