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...

WCCCEDU/twitter-commons,jsirois/commons,WCCCEDU/twitter-commons,nkhuyu/commons,WCCCEDU/twitter-commons,jsirois/commons,brutkin/commons,VaybhavSharma/commons,abel-von/commons,cevaris/commons,pombredanne/commons,nkhuyu/commons,VaybhavSharma/commons,imsut/commons,VaybhavSharma/commons,ericzundel/commons,WCCCEDU/twitter-commons,ericzundel/commons,jsirois/commons,pombredanne/commons,atollena/commons,Yasumoto/commons,nkhuyu/commons,brutkin/commons,jsirois/commons,abel-von/commons,cevaris/commons,Yasumoto/commons,WCCCEDU/twitter-commons,brutkin/commons,brutkin/commons,atollena/commons,abel-von/commons,cevaris/commons,cevaris/commons,atollena/commons,Yasumoto/commons,nkhuyu/commons,ericzundel/commons,brutkin/commons,nkhuyu/commons,pombredanne/commons,VaybhavSharma/commons,Yasumoto/commons,abel-von/commons,atollena/commons,jsirois/commons,ericzundel/commons,atollena/commons,cevaris/commons,imsut/commons,pombredanne/commons,VaybhavSharma/commons,Yasumoto/commons,ericzundel/commons,imsut/commons,pombredanne/commons,abel-von/commons