Adjust requirements_update_strategy to fix pub crashes (#303)

* Use strings for requirements update strategy

Dependabot's pub package manager only supports string values for requirements_update_strategy, not symbols

Adjust update script to pass in strings instead of symbols so that pub can work

* Special case the version strategy for pub

Add a special case for pub version strategies instead of using strings for all package managers, for conservatism

src/script/update-script.rb

@@ -138,7 +138,7 @@
   requirements_update_strategy_raw = ENV["DEPENDABOT_VERSIONING_STRATEGY"] || "auto"
   $options[:requirements_update_strategy] = VERSIONING_STRATEGIES.fetch(requirements_update_strategy_raw)
 
-  # For npm_and_yarn, we must correct the strategy to one allowed
+  # For npm_and_yarn & composer, we must correct the strategy to one allowed
   # https://github.com/dependabot/dependabot-core/blob/5ec858331d11253a30aa15fab25ae22fbdecdee0/npm_and_yarn/lib/dependabot/npm_and_yarn/update_checker/requirements_updater.rb#L18-L19
   # https://github.com/dependabot/dependabot-core/blob/5926b243b2875ad0d8c0a52c09210c4f5f274c5e/composer/lib/dependabot/composer/update_checker/requirements_updater.rb#L23-L24
   if $package_manager == "npm_and_yarn" || $package_manager == "composer"
@@ -147,6 +147,19 @@
       $options[:requirements_update_strategy] = :bump_versions
     end
   end
+
+  # For pub, we also correct the strategy
+  # https://github.com/dependabot/dependabot-core/blob/ca9f236591ba49fa6e2a8d5f06e538614033a628/pub/lib/dependabot/pub/update_checker.rb#L110
+  if $package_manager == "pub"
+    strategy = $options[:requirements_update_strategy]
+    if strategy == :auto
+      $options[:requirements_update_strategy] = nil
+    elsif strategy == :lockfile_only
+      $options[:requirements_update_strategy] = "bump_versions"
+    else
+      $options[:requirements_update_strategy] = strategy.to_s
+    end
+  end
 end
 
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