Used cached compiled map in LintUnused
Used cached compiled map in LintUnused

Linting unused keys was adding a significant overhead to sbt project
loading because Def.compiled is so slow. It was around 4 seconds in the
sbt project on my computer.

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