fix: Ensure Rack Events Handler Exists

[arielvalentin]

Fixes #518

When attempting to use the Rack Events Handler via ActionPack, the install process raises the following error:

Failure/Error: Rails.application.initialize!

NameError:
  uninitialized constant OpenTelemetry::Instrumentation::Rack::Middlewares::EventHandler

              [::Rack::Events, [OpenTelemetry::Instrumentation::Rack::Middlewares::EventHandler.new]]
                                                                                 ^^^^^^^^^^^^^^
# /usr/local/rvm/gems/ruby-3.1.4/gems/opentelemetry-instrumentation-rack-0.23.1/lib/opentelemetry/instrumentation/rack/instrumentation.rb:44:in `middleware_args'
# /usr/local/rvm/gems/ruby-3.1.4/gems/opentelemetry-instrumentation-action_pack-0.7.0/lib/opentelemetry/instrumentation/action_pack/railtie.rb:17:in `block in <class:Railtie>'

This is due to constant value shadowing occurring in the instrumentation namespace, where the defined check in require_dependencies is not searching the root namespace and is instead searching the OpenTelemetry::Instrumentation::Rack namespace:

Rack::Events
(ruby) defined?(Rack::Events)
nil
(ruby) defined?(::Rack::Events)
"constant"

This results in the Event Handler source not being loaded.