Move lib/tasks/*.rake to tasks/*.rake

Railties loads rake files from gems' `lib/tasks` directory these days. As
such, tasks therein should be seen as public, shared, and ready for use
in consuming projects. None of the tasks we have here fit that
description, and attempting to use them (for example, in transition)
causes errors using Rake as the consuming application attempts to load
wholly inappropriate things like `gem_publisher`. Moving gem management-specific tasks to a
root `./tasks` directory makes this problem go away.