Display log when sbt loading is going to pause

Def.make could take 10099ms for 100 subprojects. This would display
logs probably for projects with more than 10 subprojects, which might
pause for a few seconds during load.

main/src/main/scala/sbt/Load.scala

@@ -154,6 +154,10 @@ object Load {
       }
       val delegates = timed("Load.apply: config.delegates", log) { config.delegates(loaded) }
       val data = timed("Load.apply: Def.make(settings)...", log) {
+        // When settings.size is 100000, Def.make takes around 10s.
+        if (settings.size > 10000) {
+          log.info(s"Resolving key references (${settings.size} settings) ...")
+        }
         Def.make(settings)(delegates, config.scopeLocal, Project.showLoadingKey(loaded))
       }
       Project.checkTargets(data) foreach sys.error