Fix warnings in util-collection project

There were warnings because of
- unused pattern match values
- deprecations
- a Xlint bug in scalac

internal/util-collection/src/main/scala/sbt/internal/util/Attributes.scala

@@ -229,7 +229,9 @@ private class BasicAttributeMap(private val backing: Map[AttributeKey[_], Any])
     }
 
   def entries: Iterable[AttributeEntry[_]] =
-    for ((k: AttributeKey[kt], v) <- backing) yield AttributeEntry(k, v.asInstanceOf[kt])
+    backing.collect {
+      case (k: AttributeKey[kt], v) => AttributeEntry(k, v.asInstanceOf[kt])
+    }
 
   private[sbt] def setCond[T](k: AttributeKey[T], opt: Option[T]): AttributeMap =
     opt match {

internal/util-collection/src/main/scala/sbt/internal/util/IDSet.scala

@@ -41,7 +41,7 @@ object IDSet {
     def +=(t: T) = { backing.put(t, Dummy); () }
     def ++=(t: Iterable[T]) = t foreach +=
     def -=(t: T) = if (backing.remove(t) eq null) false else true
-    def all = collection.JavaConversions.collectionAsScalaIterable(backing.keySet)
+    def all = collection.JavaConverters.collectionAsScalaIterable(backing.keySet)
     def toList = all.toList
     def isEmpty = backing.isEmpty
 

internal/util-collection/src/main/scala/sbt/internal/util/Settings.scala

@@ -793,7 +793,7 @@ trait Init[Scope] {
     // proper solution is for evaluate to be deprecated or for external use only and a new internal method returning Either be used
     private[this] def trapBadRef[A](run: => A): Option[A] =
       try Some(run)
-      catch { case e: InvalidReference => None }
+      catch { case _: InvalidReference => None }
 
     private[sbt] def processAttributes[B](init: B)(f: (B, AttributeMap) => B): B = a match {
       case None    => init

internal/util-collection/src/main/scala/sbt/internal/util/Signal.scala

@@ -16,7 +16,7 @@ object Signals {
       try {
         val signals = new Signals0
         signals.withHandler(signal, handler, action)
-      } catch { case e: LinkageError => Right(action()) }
+      } catch { case _: LinkageError => Right(action()) }
 
     result match {
       case Left(e)  => throw e
@@ -61,7 +61,7 @@ object Signals {
     try {
       val signals = new Signals0
       signals.supported(signal)
-    } catch { case e: LinkageError => false }
+    } catch { case _: LinkageError => false }
 }
 
 // Must only be referenced using a
@@ -70,7 +70,7 @@ object Signals {
 private final class Signals0 {
   def supported(signal: String): Boolean = {
     import sun.misc.Signal
-    try { new Signal(signal); true } catch { case e: IllegalArgumentException => false }
+    try { new Signal(signal); true } catch { case _: IllegalArgumentException => false }
   }
 
   // returns a LinkageError in `action` as Left(t) in order to avoid it being