enviornment-based enablement of zeitwerk

closes FOO-2440
flag=none

TEST PLAN:
  1) specs pass

Change-Id: I10afd058843a42ce7a89d1a99fca63029c07c09e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/277154
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

config/application.rb

@@ -43,7 +43,16 @@ Bundler.require(*Rails.groups)
 #      ...
 #      CANVAS_ZEITWERK: 1
 unless defined?(CANVAS_ZEITWERK)
-  CANVAS_ZEITWERK = (ENV['CANVAS_ZEITWERK'] == '1')
+  CANVAS_ZEITWERK = if ENV['CANVAS_ZEITWERK']
+                      (ENV['CANVAS_ZEITWERK'] == '1')
+                    elsif Rails.root && (zw_settings = ConfigFile.load("zeitwerk"))
+                      zw_settings["enabled"]
+                    else
+                      # choose to force zeitwerk on in dev/test
+                      # environments unless they override with
+                      # an env var or file
+                      !Rails.env.production?
+                    end
 end
 
 module CanvasRails