Improve structureIndex call by using Vector
Improve structureIndex call by using Vector

This call takes around 8035ms for 100 subprojects. I don't think using
Vector here had any noticeable effect.

sbt/sbt,mdedetrich/sbt,mdedetrich/sbt,sbt/sbt,xuwei-k/xsbt,Duhemm/sbt,mdedetrich/sbt,xuwei-k/xsbt,mdedetrich/sbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt,mdedetrich/sbt,Duhemm/sbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt