Adapt to change in ClassLoader in JDK 9

A package protected method was added:

    Stream<Package> packages()

The private accessor method for `val package` in our subclass
now violates the "cannot tighten access" rule.

Making it `private[this]` avoids creating an accessor.

Author: retronym

src/reflect/scala/reflect/internal/util/AbstractFileClassLoader.scala

@@ -92,7 +92,7 @@ class AbstractFileClassLoader(val root: AbstractFile, parent: ClassLoader)
     }
   }
 
-  private val packages = mutable.Map[String, Package]()
+  private[this] val packages = mutable.Map[String, Package]()
 
   override def definePackage(name: String, specTitle: String, specVersion: String, specVendor: String, implTitle: String, implVersion: String, implVendor: String, sealBase: URL): Package = {
     throw new UnsupportedOperationException()