RFR 8156497: Add jar tool support for Multi-release modular JARs

[Chris Hegarty]

What do you get if you mix JEP 261 [1] with JEP 238 [2]?
A Multi-release modular JAR.

This issue proposes to add support to the jar tool for creating and
updating modular JAR files with an optional module-info.class in the
versioned section.

MRJARs are intended to target multiple releases of the Java platform, so
it seems reasonable for a Multi-release modular JAR ( MRMJAR ) to be
able to declare a different set of dependencies on modules that are part
of the Java platform. The reasoning here is that these are
implementation details rather than parts of a module's API surface, and
that one may well want to change them as the JDK itself evolves. The
runtime already has support for loading from the versioned section.

Specifically, a versioned module descriptor must be identical to the
root module descriptor, with two exceptions:

- A versioned descriptor can have different non-public `requires`
clauses of platform ( `java.*` and `jdk.*` ) modules, and

- A versioned descriptor can have different `uses` clauses, even of
service types defined outside of `java.*` and `jdk.*` modules.

http://cr.openjdk.java.net/~chegar/8156497.00/

Note: while there are some new test scenarios added, which give
reasonable coverage, further tests will be added later. Steve has some
additional jar tools support coming for easier creation of MRJARS. 

-Chris.

[1] http://openjdk.java.net/jeps/261
[2] http://openjdk.java.net/jeps/238