Include only bundled and default gems in stdlib jar

[headius]

In order to fix #6416 I modified the build of jruby-stdlib to include all contents of lib/ruby/gems, since previously it excluded any gems that had platform files (e.g. racc). However this introduced an issue: when building locally for dev purposes, it will include all gems installed locally. For a typical local JRuby dev environment, this may include hundreds or thousands of gems.

We need a middle ground that does not whitelist only specific gems, but also does not include gems that are not part of the bundled or default sets. It should be possible to do this based on the list of gems in the pom, provided that covers all transitive dependencies as well.

@mkristian If you are still able to help, some cleanup of this packaging would be great. We want to do a more extensive cleanup on master, as well.