* Use newer backport9 to access Java 8 StackWalker
* Add module-info for modules we require
* Copy jruby-base jar and deps into lib/modules
* Modify jruby.bash to handle unshaded jruby and deps as classpath
  or module path entries
* Restructure build for module-info.java

There's a remaining issue here to build: CallSite and Main have
sibling packages with the same name as lowercase, and that appears
to be rejected by the build currently.

Ant is no longer maintained, and as a result there's no module-
based version of its jar. We must move this out to a separate
library that can be used at build time, such as during JRuby's own
uses of the Rake ant targets.

We only have this dependency due to the @generated annotation we
add to some generated sources, but neither we nor anyone else ever
consumes that annotation.

Default scope for copy-dependencies is "test", which copies all
runtime, provided, and test dependencies. We only want runtime in
lib/modules.

This patch makes it possible to boot JRuby fully modularized. A
few notes:

* The org.jruby.base module is now open, because I was too lazy
  to individually open all of the Ruby kernel paths and there is
  no way to open them recursively from module-info.
* Ruby "kernel" sources have been moved into org.jruby.kernel to
  aid in accessing them in a module-friendly way. This does,
  unfortunately, result in paths like
  org/jruby/kernel/kernel/kernel.rb. An alternative name may be in
  order.

Using lookup() appears to return a Lookup object that is stricter
about access restrictions from a parent module to a child
classloader with no module. In this case, the variables in the
reified object classes are to be wired up in accessors. Using the
lookup() version results in an error:

java.lang.IllegalAccessException: symbolic reference class is not accessible: class org.jruby.gen.RubyObject5, from class org.jruby.specialized.RubyObjectSpecializer (module org.jruby.base)

Without this you cannot incrementally build. For some reason clean
build still works.

* Drop another Ant reference in favor of Java 11 Files.readString.
* Remove unnecessary and absent java.xml.ws.annotation module.

This was added as a dependency prior to modularization, but the
modularization work was rebased on top of a more recent version.