fail the build on unused third party jars [LUCENE-4017]

#4884 removed 4 useless jars from benchmark/lib.

How many others do we have in this boat?

Can't we do something like http://sixlegs.com/blog/java/depfind.html
on each module, and if there is no path thru the dependency graph from
its build/xyz.jar to one in lib/, then fail (with a way to have exceptions
for only-reflected stuff)

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Migrated from [LUCENE-4017](https://issues.apache.org/jira/browse/LUCENE-4017) by Robert Muir (@rmuir)

Start by auditing benchmark/lib and the lib directories for each module, then trace each module's build/xyz.jar dependencies as described. Clarify how dependency-graph failures and reflection-only exceptions should be represented before implementation; done means unused jars are detected and intentional exceptions do not fail the build.

Written by the indexing model from the issue text.