Previously whenever we see a new type of Java object, we both set
up the proxy class AND set the constant. If that object's class
is not from the typical Java integration classloader hierarchy, it
will pollute the Java:: module with a normally inaccessible entry.

This change only sets up the constant when it is accessed through
the normal package reference, rather than when first seen.

Fixes jruby#8156

In order to avoid polluting the Java:: namespace with classes from
other classloader hierarchies, we are modifying the proxy class
setup process to not eagerly set a constant for newly-encountered
classes. This property controls the behavior, defaulting to true
for JRuby 9.4.x but likely to switch to false in a future release
(probably JRuby 10).

The constants will still be defined if accessed directly, as
before. We just won't define them when we happen to see an object
with an unfamiliar type.

See jruby#8208 and the bug it fixed jruby#8156.