Change Gradle to use java-library plugin

Change Gradle to use java-library plugin

This plugin is specifically built for libraries. The major difference to the regular java plugin is that is allows defining dependencies as part of the api or implementation. Both are used by the project at compile/runtime, but only api dependencies are made available to dependent projects.

In our current setup, this doesn't matter much. We deploy to maven central using pre-built POMs. It's more of a future-proofing, and it makes it a little bit clearer which gradle projects actually require which dependencies.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1887650 13f79535-47bb-0310-9956-ffa450edef68