Re-examine upgradeable modules

This task is for tracking purposes.

From the Project Jigsaw: Goals & Requirements document:

"Upgradeable modules — If a JDK module implements an endorsed standard or standalone technology then it must be possible to use a version of that module from a later release in any phase, i.e., at compile time, build time, or run time."

This task need to be fleshed out further and sub-tasks created for the specific areas. For example, we need to identify the Java SE modules that are allowed to be overridden, we need both javac and the runtime to support, we need to see how this relates to dynamic configuration (although this already works in our prototypes builds by way of the "before finder").

For now we assume that this feature is separate to -Xoverride (the module replacement to -Xbootclasspath/a) as this option is about replacing classes in existing modules.