Add support to alias included default task

This PR addresses the issue brought up in #661. I've also updated the docs and added test coverage!

Included Taskfiles that declare a default task will be aliased automatically to their namespace only if there is not a conflicting task in the parent Taskfile.

version: '3'
includes:
  docker: ./DockerTasks.yml
tasks:
  deploy:
    - task: docker # will run docker:default

...

version: '3'
includes:
  docker: ./DockerTasks.yml
tasks:
  docker: # declaring `docker` will prevent the above behavior
    cmds:
      - task: docker:default # but the default task can still be used