[Style] Replace map(…).flatten with flat_map

lib/cocoapods/installer/target_installer/aggregate_target_installer.rb

@@ -114,9 +114,9 @@ module Pod
         library_targets = target.pod_targets.reject do |pod_target|
           pod_target.should_build? && pod_target.requires_frameworks?
         end
-        file_accessors = library_targets.map(&:file_accessors).flatten
-        resource_paths = file_accessors.map { |accessor| accessor.resources.flatten.map { |res| res.relative_path_from(project.path.dirname) } }.flatten
-        resource_bundles = file_accessors.map { |accessor| accessor.resource_bundles.keys.map { |name| "${BUILT_PRODUCTS_DIR}/#{name}.bundle" } }.flatten
+        file_accessors = library_targets.flat_map(&:file_accessors)
+        resource_paths = file_accessors.flat_map { |accessor| accessor.resources.flat_map { |res| res.relative_path_from(project.path.dirname) } }
+        resource_bundles = file_accessors.flat_map { |accessor| accessor.resource_bundles.keys.map { |name| "${BUILT_PRODUCTS_DIR}/#{name}.bundle" } }
         resources = []
         resources.concat(resource_paths)
         resources.concat(resource_bundles)