RFR: 8152897: refactor ToolBox to allow reduced documented dependencies

[Jonathan Gibbons]

JBS: https://bugs.openjdk.java.net/browse/JDK-8152897
Review: http://cr.openjdk.java.net/~jjg/8152897/webrev.00/index.html

This is a test-only change, which updates the ToolBox test library from 
being a single top-level class in the unnamed package to a small set of 
classes in a new named package. Although this does increase the amount 
of overhead in the test description header, it also allows us to be more 
precise in the test dependencies, such that we can reduce otherwise 
unnecessary compilation dependencies. This will also make it more 
practical to extend the set of supported ToolBox tasks, such as for 
jmod, jlink etc, so that only tests that would want to use such tasks 
would incur any need to build the tasks.

The update is primarily for javac tests,  but also includes changes for 
tests for jshell and sjavac which also use the ToolBox test library.

There is no significant change to the functionality of any tests. The 
build dependencies have been verified by running each test separately 
within jtreg, using a clean JTwork/classes directory for each test.

-- Jon