fix getting lockfile issue.

npm_and_yarn/lib/dependabot/npm_and_yarn/package_manager.rb

@@ -160,7 +160,7 @@ def detect_package_manager
       def name_from_lockfiles
         # We prioritize the package manager that has a lockfile
         PACKAGE_MANAGER_CLASSES.each_key do |manager_name| # iterates keys in order as defined in the hash
-          return manager_name.to_s if @lockfiles[manager_name]
+          return manager_name.to_s if @lockfiles[manager_name.to_sym]
         end
         nil
       end