Fix memory leak in CI config includes entry

What does this MR do and why?

This backports !122462 (merged) to 15-10-stable-ee.

In GitLab 11.9, gitlab-foss!24098 (merged) introduced validation of include keywords. However, it quietly introduced a memory leak: any time a new include entry was instantiated, a proc would be added to the aspects class variable. As the list grew, the time taken to process other include keywords would grow.

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
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• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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lib/gitlab/ci/config/entry/includes.rb

@@ -7,7 +7,7 @@ module Entry
         ##
         # Entry that represents a list of include.
         #
-        class Includes < ::Gitlab::Config::Entry::Node
+        class Includes < ::Gitlab::Config::Entry::ComposableArray
           include ::Gitlab::Config::Entry::Validatable
 
           validations do
@@ -23,16 +23,8 @@ class Includes < ::Gitlab::Config::Entry::Node
             end
           end
 
-          def self.aspects
-            super.append -> do
-              @config = Array.wrap(@config)
-
-              @config.each_with_index do |config, i|
-                @entries[i] = ::Gitlab::Config::Entry::Factory.new(Entry::Include)
-                                .value(config || {})
-                                .create!
-              end
-            end
+          def composable_class
+            Entry::Include
           end
         end
       end