Allow each plugin to have a matching directory full of dependency jars

I am proposing this separately in case there are objections or alternative proposals.

Right now, the recommendation is for plugins to build a jar-with-dependencies. This will continue to work, but is inconsistent with the base system and runs into possible LGPL conflicts.

It is also possible to just merge plugin dependencies into the main server library directory. But that is a maintenance headache.

This branch allows each plugins jar to have a matching subdirectory whose contents are also included in the classpath. The subdirectory has the same name, less the `.jar` suffix. It could be named after the plugin itself, which is shorter, but potentially more opaque.

Note that all plugins together form a single class path. Nothing is proposed concerning version or other conflicts among plugins or with the base system. One really needs something like OSGi for that.

No tests / uses are included here. An upcoming branch will split out some external interfaces into plugins.

This branch also adjusts the development and Windows policy files to grant all to plugins. Previously only the Unix packaging one did this.