Simplify MultipleRootedProducts._get_products_for_root by using setdefault
Simplify MultipleRootedProducts._get_products_for_root by using setdefault

Testing Done:
./pants goal test tests/python/pants_test:all

Reviewed at https://rbcommons.com/s/twitter/r/851/

dturner-tw/pants,lahosken/pants,peiyuwang/pants,dgomez10/xanon,mateor/pants,peiyuwang/pants,qma/pants,pantsbuild/pants,landism/pants,square/pants,digwanderlust/pants,baroquebobcat/pants,wisechengyi/pants,tejal29/pants,tdyas/pants,slyphon/pants,Gabriel439/pants,manasapte/pants,kwlzn/pants,baroquebobcat/pants,baroquebobcat/pants,kslundberg/pants,cevaris/pants,dbentley/pants,peiyuwang/pants,landism/pants,foursquare/pants,kwlzn/pants,ericzundel/pants,areitz/pants,fkorotkov/pants,peiyuwang/pants,manasapte/pants,sameerparekh/pants,15Dkatz/pants,lahosken/pants,sid-kap/pants,ity/pants,manasapte/pants,tdyas/pants,jsirois/pants,UnrememberMe/pants,di0spyr0s/pants,jessrosenfield/pants,ity/pants,laurentgo/pants,scode/pants,fkorotkov/pants,tdyas/pants,TansyArron/pants,sameerparekh/pants,benjyw/pants,Gabriel439/pants,foursquare/pants,megaserg/pants,kwlzn/pants,foursquare/pants,sid-kap/pants,scode/pants,areitz/pants,dturner-tw/pants,benjyw/pants,digwanderlust/pants,square/pants,UnrememberMe/pants,foursquare/pants,benjyw/pants,megaserg/pants,landism/pants,pantsbuild/pants,twitter/pants,jtrobec/pants,sid-kap/pants,TansyArron/pants,wisechengyi/pants,areitz/pants,mateor/pants,Gabriel439/pants,twitter/pants,laurentgo/pants,dturner-tw/pants,TansyArron/pants,gmalmquist/pants,baroquebobcat/pants,fkorotkov/pants,kslundberg/pants,tejal29/pants,foursquare/pants,pombredanne/pants,qma/pants,kslundberg/pants,twitter/pants,tejal29/pants,jtrobec/pants,foursquare/pants,ericzundel/pants,jessrosenfield/pants,ericzundel/pants,jessrosenfield/pants,UnrememberMe/pants,baroquebobcat/pants,kwlzn/pants,pgroudas/pants,qma/pants,TansyArron/pants,scode/pants,fkorotkov/pants,megaserg/pants,UnrememberMe/pants,wisechengyi/pants,15Dkatz/pants,qma/pants,tejal29/pants,tdyas/pants,scode/pants,di0spyr0s/pants,laurentgo/pants,wisechengyi/pants,15Dkatz/pants,sameerparekh/pants,jsirois/pants,wisechengyi/pants,UnrememberMe/pants,slyphon/pants,pgroudas/pants,dgomez10/xanon,wisechengyi/pants,scode/pants,areitz/pants,dgomez10/xanon,UnrememberMe/pants,cevaris/pants,foursquare/pants,baroquebobcat/pants,wisechengyi/pants,jsirois/pants,slyphon/pants,manasapte/pants,jessrosenfield/pants,gmalmquist/pants,ity/pants,sid-kap/pants,tdyas/pants,twitter/pants,lahosken/pants,dgomez10/xanon,qma/pants,15Dkatz/pants,Gabriel439/pants,ericzundel/pants,ity/pants,UnrememberMe/pants,gmalmquist/pants,pombredanne/pants,dgomez10/xanon,dbentley/pants,digwanderlust/pants,slyphon/pants,Gabriel439/pants,di0spyr0s/pants,pombredanne/pants,scode/pants,baroquebobcat/pants,baroquebobcat/pants,jtrobec/pants,megaserg/pants,wisechengyi/pants,dgomez10/xanon,UnrememberMe/pants,twitter/pants,Gabriel439/pants,15Dkatz/pants,benjyw/pants,peiyuwang/pants,digwanderlust/pants,kslundberg/pants,jessrosenfield/pants,slyphon/pants,ericzundel/pants,kslundberg/pants,kslundberg/pants,pantsbuild/pants,landism/pants,jtrobec/pants,ity/pants,pantsbuild/pants,laurentgo/pants,gmalmquist/pants,twitter/pants,pombredanne/pants,cevaris/pants,landism/pants,dbentley/pants,peiyuwang/pants,cevaris/pants,lahosken/pants,digwanderlust/pants,twitter/pants,ericzundel/pants,digwanderlust/pants,di0spyr0s/pants,dbentley/pants,sid-kap/pants,mateor/pants,tdyas/pants,15Dkatz/pants,dgomez10/xanon,dgomez10/xanon,Gabriel439/pants,slyphon/pants,areitz/pants,jessrosenfield/pants,sameerparekh/pants,landism/pants,peiyuwang/pants,di0spyr0s/pants,lahosken/pants,pombredanne/pants,kwlzn/pants,15Dkatz/pants,scode/pants,benjyw/pants,qma/pants,fkorotkov/pants,pantsbuild/pants,pgroudas/pants,dgomez10/xanon,ericzundel/pants,megaserg/pants,sameerparekh/pants,TansyArron/pants,twitter/pants,gmalmquist/pants,square/pants,dturner-tw/pants,tdyas/pants,lahosken/pants,dbentley/pants,pantsbuild/pants,di0spyr0s/pants,slyphon/pants,cevaris/pants,sid-kap/pants,ity/pants,qma/pants,pantsbuild/pants,landism/pants,tejal29/pants,tdyas/pants,wisechengyi/pants,manasapte/pants,kwlzn/pants,gmalmquist/pants,pgroudas/pants,15Dkatz/pants,ity/pants,di0spyr0s/pants,lahosken/pants,jtrobec/pants,areitz/pants,pgroudas/pants,gmalmquist/pants,mateor/pants,UnrememberMe/pants,manasapte/pants,foursquare/pants,peiyuwang/pants,dbentley/pants,megaserg/pants,laurentgo/pants,megaserg/pants,fkorotkov/pants,pombredanne/pants,fkorotkov/pants,cevaris/pants,kslundberg/pants,dgomez10/xanon,areitz/pants,mateor/pants,cevaris/pants,twitter/pants,benjyw/pants,jessrosenfield/pants,dbentley/pants,sid-kap/pants,pgroudas/pants,TansyArron/pants,mateor/pants,dturner-tw/pants,tdyas/pants,mateor/pants,sameerparekh/pants,dturner-tw/pants,pombredanne/pants,sameerparekh/pants,benjyw/pants,dgomez10/xanon,kwlzn/pants,ericzundel/pants,laurentgo/pants,square/pants,lahosken/pants,TansyArron/pants,foursquare/pants,mateor/pants,jtrobec/pants,baroquebobcat/pants,dturner-tw/pants,fkorotkov/pants,manasapte/pants,jtrobec/pants,landism/pants