Support extra module references in the REPL

By default, scala-compiler.jar is in the bootclasspath, which gives us a
`null` when we call `classOf[IMain].getClassLoader`. That value is used
as the parent class loader of the classloader that evals code in the REPL.

Under JDK9, this breaks lookup of classes on the module path:

```
 scala -J--add-modules=java.compiler -J--add-exports=jdk.jdeps/com.sun.tools.javap=ALL-UNNAMED
Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
Type in expressions for evaluation. Or try :help.

scala> new com.sun.tools.javap.JavapTask();
java.lang.NoClassDefFoundError: com/sun/tools/javap/JavapTask
  at java.base/java.lang.Class.getDeclaredMethods0(Native Method)
  at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:2948)
 ```

 A workaround is to use `-nobootcp`.

 This commit uses the system classloader as the parent in place of the null classloader
 to make this work in both cases.

 ```
 qscala -J--add-modules=java.compiler -J--add-exports=jdk.jdeps/com.sun.tools.javap=ALL-UNNAMED
 Welcome to Scala 2.12.2-20161208-165912-3de1c0c (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
 Type in expressions for evaluation. Or try :help.

 scala> new com.sun.tools.javap.JavapTask();
 res0: com.sun.tools.javap.JavapTask = com.sun.tools.javap.JavapTask@1f1cddf3

 scala>
 ```

 Note that the `:javap` command still requires `-nobootcp` because code in the REPL implements
 an interface in a the `java.compiler` module.

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