Let Action Cable's autoloader ignore the version file

This gem is organized in a particular way with regard to versioning.

On one hand, ActionCable::VERSION is defined in action_cable/gem_version.rb, but
that file does not follow Zeitwerk conventions, so we ignore it.

Addtionally, action_cable/gem_version.rb defines ActionCable.gem_version, and we
need to eager load it to have that method available for client code.

On the other hand, there is also action_cable/version.rb, which loads
action_cable/gem_version.rb. That file follows the conventions because by
loading it, the expected constant gets defined as a side-effect.

All in all, the setup that we had technically works, but I feel autoloading
action_cable/version.rb makes you think too much about the interactions between
these two files.

I think this simplification is easier to understand.

actioncable/lib/action_cable.rb

@@ -29,8 +29,21 @@
 
 Zeitwerk::Loader.for_gem.tap do |loader|
   loader.ignore(
-    "#{__dir__}/rails", # Contains generators, templates, docs, etc.
+    # Contains generators, templates, docs, etc.
+    "#{__dir__}/rails",
+
+    # These two files set things up in a special way.
+    #
+    # action_cable/gem_version.rb is not autoloadable, because it does not
+    # define the expected constant. action_cable/version.rb is, and it defines
+    # the expected constant as a side-effect of loading the gem_version file.
+    #
+    # We could configure the autoloader accordingly, but loading the files
+    # manually makes you think less.
     "#{__dir__}/action_cable/gem_version.rb",
+    "#{__dir__}/action_cable/version.rb",
+
+    # Defines Rails.deprecator, which is not autoloadable.
     "#{__dir__}/action_cable/deprecator.rb",
   )
 
@@ -46,6 +59,7 @@
 # :markup: markdown
 # :include: ../README.md
 module ActionCable
+  require_relative "action_cable/gem_version"
   require_relative "action_cable/version"
   require_relative "action_cable/deprecator"