Add public api markers for core_tasks
Add public api markers for core_tasks

The following modules were reviewed and all api's were left as private. As
far as I can tell these modules are not currently used by plugins.

* pants.core_tasks.bash_completion.py
* pants.core_tasks.changed_target_tasks.py
* pants.core_tasks.clean.py
* pants.core_tasks.deferred_sources_mapper.py
* pants.core_tasks.explain_options_task.py
* pants.core_tasks.invalidate.py
* pants.core_tasks.list_goals.py
* pants.core_tasks.pantsd_kill.py
* pants.core_tasks.register.py
* pants.core_tasks.reporting_server_kill.py
* pants.core_tasks.reporting_server_run.py
* pants.core_tasks.roots.py
* pants.core_tasks.run_prep_command.py
* pants.core_tasks.targets_help.py
* pants.core_tasks.what_changed.py

Testing Done:
CI green: https://travis-ci.org/pantsbuild/pants/builds/111549646

Bugs closed: 2710, 2955

Reviewed at https://rbcommons.com/s/twitter/r/3490/

tdyas/pants,cevaris/pants,ity/pants,15Dkatz/pants,ericzundel/pants,wisechengyi/pants,ity/pants,lahosken/pants,pombredanne/pants,foursquare/pants,kwlzn/pants,peiyuwang/pants,fkorotkov/pants,dbentley/pants,benjyw/pants,tdyas/pants,peiyuwang/pants,baroquebobcat/pants,tdyas/pants,lahosken/pants,pantsbuild/pants,baroquebobcat/pants,twitter/pants,tdyas/pants,baroquebobcat/pants,lahosken/pants,landism/pants,pantsbuild/pants,kwlzn/pants,UnrememberMe/pants,foursquare/pants,ity/pants,benjyw/pants,benjyw/pants,peiyuwang/pants,foursquare/pants,jsirois/pants,manasapte/pants,pombredanne/pants,pantsbuild/pants,UnrememberMe/pants,baroquebobcat/pants,UnrememberMe/pants,ericzundel/pants,mateor/pants,mateor/pants,dbentley/pants,dbentley/pants,pombredanne/pants,fkorotkov/pants,cevaris/pants,lahosken/pants,landism/pants,ericzundel/pants,twitter/pants,pantsbuild/pants,baroquebobcat/pants,kwlzn/pants,twitter/pants,fkorotkov/pants,twitter/pants,twitter/pants,tdyas/pants,kwlzn/pants,wisechengyi/pants,15Dkatz/pants,pombredanne/pants,fkorotkov/pants,cevaris/pants,kwlzn/pants,foursquare/pants,foursquare/pants,wisechengyi/pants,ity/pants,benjyw/pants,tdyas/pants,baroquebobcat/pants,jsirois/pants,pantsbuild/pants,baroquebobcat/pants,pantsbuild/pants,manasapte/pants,landism/pants,benjyw/pants,pombredanne/pants,15Dkatz/pants,UnrememberMe/pants,landism/pants,peiyuwang/pants,mateor/pants,ericzundel/pants,tdyas/pants,tdyas/pants,peiyuwang/pants,landism/pants,gmalmquist/pants,15Dkatz/pants,lahosken/pants,cevaris/pants,cevaris/pants,twitter/pants,lahosken/pants,fkorotkov/pants,dbentley/pants,gmalmquist/pants,gmalmquist/pants,kwlzn/pants,dbentley/pants,gmalmquist/pants,dbentley/pants,cevaris/pants,dbentley/pants,ericzundel/pants,landism/pants,ity/pants,UnrememberMe/pants,mateor/pants,foursquare/pants,mateor/pants,UnrememberMe/pants,wisechengyi/pants,wisechengyi/pants,lahosken/pants,cevaris/pants,manasapte/pants,foursquare/pants,UnrememberMe/pants,twitter/pants,wisechengyi/pants,gmalmquist/pants,15Dkatz/pants,wisechengyi/pants,fkorotkov/pants,peiyuwang/pants,gmalmquist/pants,baroquebobcat/pants,benjyw/pants,manasapte/pants,wisechengyi/pants,UnrememberMe/pants,wisechengyi/pants,mateor/pants,gmalmquist/pants,peiyuwang/pants,baroquebobcat/pants,pombredanne/pants,benjyw/pants,manasapte/pants,UnrememberMe/pants,foursquare/pants,fkorotkov/pants,pantsbuild/pants,ericzundel/pants,15Dkatz/pants,kwlzn/pants,lahosken/pants,15Dkatz/pants,twitter/pants,landism/pants,tdyas/pants,ericzundel/pants,manasapte/pants,mateor/pants,foursquare/pants,ity/pants,twitter/pants,pombredanne/pants,ity/pants,manasapte/pants,mateor/pants,peiyuwang/pants,fkorotkov/pants,ericzundel/pants,15Dkatz/pants,jsirois/pants,landism/pants