Fix infinite-loop when using Jar#iterator

So far, Jar was defined as an Iterable implementation that overrides
`foreach` and then define `override def iterator = this.toList.iterator`
That was OK with the 2.12 stdlib, but stackoverflows with the 2.13 one
due to `toList` now being implemented in terms of `iterator`.

Any way, it turns out that we don't really need Jar to be an Iterable,
so the simplest fix was to just remove these methods, and add back a
`toList` for the one usesite that needed it (not very efficient, but not
any worse than before).

Author: smarter

compiler/src/dotty/tools/dotc/Driver.scala

@@ -74,10 +74,10 @@ class Driver {
       val (classPaths, classNames) = fileNames0.flatMap { name =>
         val path = Paths.get(name)
         if (name.endsWith(".jar")) {
-          new dotty.tools.io.Jar(File(name)).iterator.collect {
+          new dotty.tools.io.Jar(File(name)).toList.collect {
             case e if e.getName.endsWith(".tasty") =>
               (name, e.getName.stripSuffix(".tasty").replace("/", "."))
-          }.toList
+          }
         }
         else if (!name.endsWith(".tasty"))
           ("", name) :: Nil

compiler/src/dotty/tools/io/Jar.scala

@@ -35,7 +35,7 @@ import scala.annotation.tailrec
 // static Attributes.Name   SPECIFICATION_VENDOR
 // static Attributes.Name   SPECIFICATION_VERSION
 
-class Jar(file: File) extends Iterable[JarEntry] {
+class Jar(file: File) {
   def this(jfile: JFile) = this(File(jfile.toPath))
   def this(path: String) = this(File(path))
 
@@ -64,10 +64,9 @@ class Jar(file: File) extends Iterable[JarEntry] {
     new JarWriter(file, Jar.WManifest.apply(mainAttrs: _*).underlying)
   }
 
-  override def foreach[U](f: JarEntry => U): Unit = withJarInput { in =>
-    Iterator continually in.getNextJarEntry() takeWhile (_ != null) foreach f
+  def toList: List[JarEntry] = withJarInput { in =>
+    Iterator.continually(in.getNextJarEntry()).takeWhile(_ != null).toList
   }
-  override def iterator: Iterator[JarEntry] = this.toList.iterator
 
   def getEntryStream(entry: JarEntry): java.io.InputStream = jarFile getInputStream entry match {
     case null   => errorFn("No such entry: " + entry) ; null