This PR includes the following changes:
* Added JDK 10
* Changed the JDK version numbers according to the consensus reached
in #2284
* Added spec['java'].home and spec['java'].libs, similar to #3367
(JDK and IcedTea)
* Added a check to prevent people from installing JDK on macOS
* Set CLASSPATH for packages depending on Java (JDK and IcedTea)
* Add TODO for extending virtual packages (not currently possible)
* Add TODO for adding Java dependents to views
* Add TODO for packages which extend multiple packages (e.g. Java
and Python)
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