Update ExternalHooks to look up changed binaries

It was reported that in community builds, sometimes there was
spurious over-compilation due to invalidation of the scala library jar
(https://github.com/sbt/sbt/issues/4948). The reason for this was that
the external hooks prefills the managed cache with all of the time
stamps for the project dependencies but was not looking up any jars that
weren't in the cache. I suspect I did this because I didn't realize that
zinc also includes its own classpath in the binaries which is not
a part of the dependencyClasspath. The fix is to just add the jar to the
cache if it doesn't already exist by switching to getOrElseUpdate from
get.

I followed the steps in #4948 and published a version of sbt locally
with this change and the spurious re-builds stopped.

main/src/main/scala/sbt/internal/ExternalHooks.scala

@@ -101,7 +101,7 @@ private[sbt] object ExternalHooks {
       override def changedBinaries(previousAnalysis: CompileAnalysis): Option[Set[File]] = {
         Some(previousAnalysis.readStamps.getAllBinaryStamps.asScala.flatMap {
           case (file, stamp) =>
-            managedCache.get(file.toPath) match {
+            managedCache.getOrElseUpdate(file.toPath, FileStamper.LastModified) match {
               case Some(cachedStamp) if equiv(cachedStamp.stamp, stamp) => None
               case _ =>
                 javaHome match {