[Fix rubocop#2044] Fix unintended document changes caused by default …

…Rake task

This PR fixes unintended document changes caused by  default Rake task.

## Problem

The default task causes `RuboCop::Config` to end up in an unexpected state when
`internal_investigation` task is executed before `generate_cops_documentation` task.
This is due to the fact that plugins are already loaded when RuboCop is executed as
part of `internal_investigation`.

## Solution

This change preemptively determines whether configuration adjustments are necessary
based on whether `rubocop-rspec` plugin has already been loaded.

## Additional Information

Since this logic should ideally be encapsulated within `CopsDocumentationGenerator`,
it is expected to become unnecessary in the future once the API design and implementation of
`CopsDocumentationGenerator` class are complete. For now, this serves as a temporary workaround.

Fixes rubocop#2044.

tasks/cops_documentation.rake

@@ -12,7 +12,11 @@ end
 
 desc 'Generate docs of all cops departments'
 task generate_cops_documentation: :yard_for_generate_documentation do
-  RuboCop::ConfigLoader.inject_defaults!("#{__dir__}/../config/default.yml")
+  plugins = RuboCop::ConfigLoader.default_configuration.loaded_plugins
+  plugin_names = plugins.map { |plugin| plugin.about.name }
+  unless plugin_names.include?('rubocop-rspec')
+    RuboCop::ConfigLoader.inject_defaults!("#{__dir__}/../config/default.yml")
+  end
 
   generator = CopsDocumentationGenerator.new(
     departments: %w[RSpec]