SI-7410 REPL uses improved tools.jar locator

SI-7410 REPL uses improved tools.jar locator
    
The logic in partest for snooping around for tools.jar
is moved to PathResolver, and ILoop uses it from there.
    
If JAVA_HOME is toolless, check out java.home.
    
The use case was that Ubuntu installed with `java` at
version 6 and `javac` at version 7; it's easy to wind
up with JAVA_HOME pointing at the version 6 JRE, as
I discovered.  It's confusing when that happens.

I hate to say it, but several months ago @paulp asked me to refactor this out of repl to a place partest could use it, then partest got the cadillac, so this ports it back to repl if @paulp reviews it and thinks it's worthwhile.

Also, @paulp is most likely to dislike or have a better suggestion for the second commit, for legibility of debug strings, which is something one needs and uses all the time.