RFR 8158131: Nashorn should not use jdk.internal.module.Modules API

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8158131/ for
https://bugs.openjdk.java.net/browse/JDK-8158131

This code cleanup is to avoid Nashorn's use of JDK internal class
jdk.internal.module.Modules.

With this change, nashorn uses java.lang.reflect.Layer public API to
create single Module layers as needed.  And nashorn generates/loads code
into appropriate modules (which calls java.lang.reflect.Module.addReads,
.addExports public APIs) to add module export/read edges as needed.

-Sundar