Be more conservative when injecting sibling resource dirs.
Be more conservative when injecting sibling resource dirs.

Two changes:

1. Prevent multiple bases that are siblings from injecting the
   same 'resources' sibling multiple times.

2. Only inject if the dir exists.

This mitigates a problem that was exposed by a bug in our
BUILD.bootstrap, whereby we hadn't registered scala_tests in
our tests/ source root. In this case the base is set to the
target's parent dir...

(sapling split of 1dc090da8f888474a97b24ca707ef714b18dc899)
TansyArron/pants,laurentgo/pants,gmalmquist/pants,sameerparekh/pants,pombredanne/pants,sameerparekh/pants,pgroudas/pants,wisechengyi/pants,slyphon/pants,mateor/pants,pgroudas/pants,lahosken/pants,UnrememberMe/pants,digwanderlust/pants,jtrobec/pants,gmalmquist/pants,peiyuwang/pants,foursquare/pants,lahosken/pants,foursquare/pants,dgomez10/xanon,baroquebobcat/pants,scode/pants,kwlzn/pants,laurentgo/pants,peiyuwang/pants,areitz/pants,jtrobec/pants,cevaris/pants,gmalmquist/pants,slyphon/pants,areitz/pants,benjyw/pants,cevaris/pants,peiyuwang/pants,15Dkatz/pants,landism/pants,laurentgo/pants,tejal29/pants,di0spyr0s/pants,kwlzn/pants,digwanderlust/pants,pombredanne/pants,peiyuwang/pants,kwlzn/pants,pantsbuild/pants,megaserg/pants,ericzundel/pants,kwlzn/pants,baroquebobcat/pants,sameerparekh/pants,pgroudas/pants,megaserg/pants,ity/pants,mateor/pants,pantsbuild/pants,slyphon/pants,wisechengyi/pants,jsirois/pants,pantsbuild/pants,ericzundel/pants,foursquare/pants,scode/pants,mateor/pants,twitter/pants,Gabriel439/pants,gmalmquist/pants,lahosken/pants,jtrobec/pants,mateor/pants,fkorotkov/pants,manasapte/pants,kslundberg/pants,dbentley/pants,lahosken/pants,dgomez10/xanon,tdyas/pants,lahosken/pants,megaserg/pants,Gabriel439/pants,baroquebobcat/pants,dbentley/pants,wisechengyi/pants,square/pants,sid-kap/pants,fkorotkov/pants,pgroudas/pants,fkorotkov/pants,twitter/pants,twitter/pants,digwanderlust/pants,TansyArron/pants,qma/pants,lahosken/pants,ericzundel/pants,gmalmquist/pants,foursquare/pants,ity/pants,landism/pants,Gabriel439/pants,tejal29/pants,jtrobec/pants,megaserg/pants,15Dkatz/pants,TansyArron/pants,foursquare/pants,dbentley/pants,laurentgo/pants,manasapte/pants,baroquebobcat/pants,dgomez10/xanon,Gabriel439/pants,sameerparekh/pants,megaserg/pants,sid-kap/pants,mateor/pants,gmalmquist/pants,landism/pants,areitz/pants,kslundberg/pants,jsirois/pants,benjyw/pants,15Dkatz/pants,kwlzn/pants,ericzundel/pants,kwlzn/pants,pantsbuild/pants,tejal29/pants,dgomez10/xanon,dturner-tw/pants,megaserg/pants,pantsbuild/pants,baroquebobcat/pants,manasapte/pants,wisechengyi/pants,pombredanne/pants,scode/pants,digwanderlust/pants,dgomez10/xanon,ericzundel/pants,scode/pants,jessrosenfield/pants,jtrobec/pants,dturner-tw/pants,ity/pants,slyphon/pants,UnrememberMe/pants,cevaris/pants,sid-kap/pants,mateor/pants,cevaris/pants,pombredanne/pants,UnrememberMe/pants,square/pants,sameerparekh/pants,mateor/pants,dgomez10/xanon,pgroudas/pants,landism/pants,landism/pants,ericzundel/pants,di0spyr0s/pants,fkorotkov/pants,manasapte/pants,jessrosenfield/pants,pantsbuild/pants,benjyw/pants,qma/pants,areitz/pants,qma/pants,kslundberg/pants,peiyuwang/pants,sid-kap/pants,kslundberg/pants,slyphon/pants,dturner-tw/pants,baroquebobcat/pants,dgomez10/xanon,di0spyr0s/pants,Gabriel439/pants,baroquebobcat/pants,UnrememberMe/pants,15Dkatz/pants,dgomez10/xanon,jessrosenfield/pants,twitter/pants,ity/pants,15Dkatz/pants,twitter/pants,di0spyr0s/pants,TansyArron/pants,manasapte/pants,foursquare/pants,ericzundel/pants,kslundberg/pants,areitz/pants,manasapte/pants,kslundberg/pants,kslundberg/pants,15Dkatz/pants,twitter/pants,dturner-tw/pants,peiyuwang/pants,foursquare/pants,ity/pants,dgomez10/xanon,tdyas/pants,landism/pants,cevaris/pants,sid-kap/pants,megaserg/pants,twitter/pants,ericzundel/pants,jtrobec/pants,Gabriel439/pants,jsirois/pants,tdyas/pants,di0spyr0s/pants,dbentley/pants,scode/pants,benjyw/pants,tejal29/pants,sid-kap/pants,sameerparekh/pants,dgomez10/xanon,15Dkatz/pants,UnrememberMe/pants,di0spyr0s/pants,pombredanne/pants,slyphon/pants,fkorotkov/pants,benjyw/pants,foursquare/pants,dturner-tw/pants,TansyArron/pants,qma/pants,cevaris/pants,tdyas/pants,sameerparekh/pants,UnrememberMe/pants,laurentgo/pants,Gabriel439/pants,15Dkatz/pants,benjyw/pants,tejal29/pants,jessrosenfield/pants,TansyArron/pants,lahosken/pants,UnrememberMe/pants,UnrememberMe/pants,sid-kap/pants,dbentley/pants,jessrosenfield/pants,foursquare/pants,dturner-tw/pants,scode/pants,manasapte/pants,wisechengyi/pants,peiyuwang/pants,lahosken/pants,square/pants,ity/pants,wisechengyi/pants,tdyas/pants,mateor/pants,twitter/pants,pgroudas/pants,baroquebobcat/pants,twitter/pants,digwanderlust/pants,areitz/pants,cevaris/pants,pantsbuild/pants,tdyas/pants,jtrobec/pants,areitz/pants,slyphon/pants,TansyArron/pants,wisechengyi/pants,peiyuwang/pants,fkorotkov/pants,qma/pants,square/pants,tdyas/pants,landism/pants,dturner-tw/pants,fkorotkov/pants,tdyas/pants,qma/pants,fkorotkov/pants,digwanderlust/pants,scode/pants,jessrosenfield/pants,dgomez10/xanon,tdyas/pants,wisechengyi/pants,gmalmquist/pants,wisechengyi/pants,kwlzn/pants,baroquebobcat/pants,qma/pants,pombredanne/pants,dbentley/pants,benjyw/pants,laurentgo/pants,dbentley/pants,ity/pants,UnrememberMe/pants,jessrosenfield/pants,di0spyr0s/pants,pombredanne/pants,landism/pants