This modifies the manifests of all executable jars we produce to
include the appropriate Add-Opens and Enable-Native-Access flags:

* /core's jruby-base jar, executable but without any shaded
  dependencies
* /shaded's jruby-core jar, executable and used as the main dist
  jar
* /maven/jruby-complete's fully-shaded jar

Normal JRuby use will go through our launcher and apply these
flags at the command line, but this allows these jars to be run
with -jar at the command line without producing warnings about
unopened packages or native library access.

Relates to the following issues that mention the native access
warning or our warnings about certain IO packages not being open:

* jruby#8727: JRuby being launched
  from within a Gradle plugin. Fixed by adding the opens flags, but
  probably also fixed by this commit.
* jruby#6721: Warnings about our
  native process and IO subsystems being unable to access the core
  JDK classes needed. May not be fixed by this issue, since JRuby
  appears to be used as a library, not as the main jar.
* jruby#8910: Not directly an issue
  related to these flags, but these flags will eliminate the
  warning for e.g. the SQLite JDBC driver when JRuby is not being
  run in the typical way.
* https://github.com/jruby/jruby/issues/8932: More issues with
  native IO and process management not working properly on Windows
  due to the native launcher not yet including the necessary flags.
  May be fixed by this commit if the launcher is also still loading
  JRuby on classpath and not as a module.