Previously, CLI extensions where installed into the CLI's lib
directory which meant that they were on the class path of the app
class loader. Following the change to an executable jar's packaging,
this meant that they could not see classes in the CLI and a
ClassNotFoundException would result.

This commit updates the CLI to install extensions into lib/ext and
load commands using a new ClassLoader that has all of the jars in
lib/ext on its class path and that uses the launch class loader as
its parent.

Closes gh-6615