Add tests for build-tools to make sure example plugins build stand-alone using it.
This will catch issues such as referencing files from the buildSrc directly, breaking external uses of build-tools.
This commit documents how to write a `ScriptEngine` in order to use
expert internal apis, such as using Lucene directly to find index term
statistics. These documents prepare the way to remove both native
scripts and IndexLookup.
The example java code is actually compiled and tested under a new gradle
subproject for example plugins. This change does not yet breakup
jvm-example into the new examples dir, which should be done separately.
relates #19359
relates #19966
Alpar Torok
Ryan Ernst