[Style] Use flat_map instead of map(:…).flatten

lib/cocoapods/installer/target_installer/pod_target_installer.rb

@@ -24,7 +24,7 @@ module Pod
             create_info_plist_file
             create_module_map
             create_umbrella_header do |generator|
-              generator.imports += target.file_accessors.map(&:public_headers).flatten.map(&:basename)
+              generator.imports += target.file_accessors.flat_map(&:public_headers).map(&:basename)
             end
           end
           create_prefix_header

lib/cocoapods/target/aggregate_target.rb

@@ -104,7 +104,7 @@ module Pod
       result = {}
       user_build_configurations.keys.each do |build_configuration|
         result[build_configuration] = pod_targets_for_build_configuration(build_configuration).
-          map(&:specs).flatten
+          flat_map(&:specs)
       end
       result
     end