Turn off flat classpath by default, mark one of its classes as sealed

This commit addresses code review comments.

The flat classpath is no longer the default classpath representation.
It was the default one just for the test purposes. For now it's not
desirable to make it permanently the default representation.

ZipAndJarFileLookupFactory is marked as sealed - it should help to
limit the ways of creating flat classpath instances for zips and jars.

Author: mpociecha

src/compiler/scala/tools/nsc/classpath/ZipAndJarFileLookupFactory.scala

@@ -18,7 +18,7 @@ import FileUtils._
  * It allows us to e.g. reduce significantly memory used by PresentationCompilers in Scala IDE
  * when there are a lot of projects having a lot of common dependencies.
  */
-trait ZipAndJarFileLookupFactory {
+sealed trait ZipAndJarFileLookupFactory {
 
   private val cache = collection.mutable.Map.empty[AbstractFile, FlatClassPath]
 

src/compiler/scala/tools/nsc/settings/ScalaSettings.scala

@@ -202,7 +202,7 @@ trait ScalaSettings extends AbsScalaSettings
   val YmethodInfer    = BooleanSetting    ("-Yinfer-argument-types", "Infer types for arguments of overriden methods.")
   val etaExpandKeepsStar = BooleanSetting ("-Yeta-expand-keeps-star", "Eta-expand varargs methods to T* rather than Seq[T].  This is a temporary option to ease transition.").withDeprecationMessage(removalIn212)
   val inferByName     = BooleanSetting    ("-Yinfer-by-name", "Allow inference of by-name types. This is a temporary option to ease transition. See SI-7899.").withDeprecationMessage(removalIn212)
-  val YclasspathImpl  = ChoiceSetting     ("-YclasspathImpl", "implementation", "Choose classpath scanning method.", List(ClassPathRepresentationType.Recursive, ClassPathRepresentationType.Flat), ClassPathRepresentationType.Flat)
+  val YclasspathImpl  = ChoiceSetting     ("-YclasspathImpl", "implementation", "Choose classpath scanning method.", List(ClassPathRepresentationType.Recursive, ClassPathRepresentationType.Flat), ClassPathRepresentationType.Recursive)
   val YdisableFlatCpCaching  = BooleanSetting    ("-YdisableFlatCpCaching", "Do not cache flat classpath representation of classpath elements from jars across compiler instances.")
 
   val YvirtClasses    = false // too embryonic to even expose as a -Y //BooleanSetting    ("-Yvirtual-classes", "Support virtual classes")