Extract `add_file` method.

Originally from 5121dff5

lib/cocoapods/sandbox/headers_store.rb

@@ -71,18 +71,40 @@ module Pod
       # @return [Array<Pathname>]
       #
       def add_files(namespace, relative_header_paths, platform)
+        relative_header_paths.map do |relative_header_path|
+          add_file(namespace, relative_header_path, relative_header_path.basename, platform)
+        end
+      end
+
+      # Adds a header to the directory under different name.
+      #
+      # @param  [Pathname] namespace
+      #         the path where the header file should be stored relative to the
+      #         headers directory.
+      #
+      # @param  [Pathname] relative_header_path
+      #         the path of the header file relative to the Pods project
+      #         (`PODS_ROOT` variable of the xcconfigs).
+      #
+      # @param  [String] final_name
+      #         the name under which the file should be available in the
+      #         headers directory.
+      #
+      # @note   This method adds the file to the search paths.
+      #
+      # @return [Pathname]
+      #
+      def add_file(namespace, relative_header_path, final_name, platform)
         add_search_path(namespace, platform)
         namespaced_path = root + namespace
         namespaced_path.mkpath unless File.exist?(namespaced_path)
 
-        relative_header_paths.map do |relative_header_path|
-          absolute_source = (sandbox.root + relative_header_path)
-          source = absolute_source.relative_path_from(namespaced_path)
-          Dir.chdir(namespaced_path) do
-            FileUtils.ln_sf(source, relative_header_path.basename)
-          end
-          namespaced_path + relative_header_path.basename
+        absolute_source = (sandbox.root + relative_header_path)
+        source = absolute_source.relative_path_from(namespaced_path)
+        Dir.chdir(namespaced_path) do
+          FileUtils.ln_sf(source, final_name)
         end
+        namespaced_path + relative_header_path.basename
       end
 
       # Adds an header search path to the sandbox.