[PodTargetInstaller] Preserve header_mappings_dir in umbrella header imports

When preserving the directory structure inside a projects header_mappings_dir,
the file structure also needs to be taken into account when generating the
umbrella header for the Pod target.

Instead of the basename, use the relative path in the header_mappings_dir, if
one is set for the Pod.

lib/cocoapods/installer/target_installer/pod_target_installer.rb

@@ -26,7 +26,13 @@ module Pod
               generator.private_headers += target.file_accessors.flat_map(&:private_headers).map(&:basename)
             end
             create_umbrella_header do |generator|
-              generator.imports += target.file_accessors.flat_map(&:public_headers).map(&:basename)
+              if header_mappings_dir
+                generator.imports += target.file_accessors.flat_map(&:public_headers).map do |pathname|
+                  pathname.relative_path_from(header_mappings_dir)
+                end
+              else
+                generator.imports += target.file_accessors.flat_map(&:public_headers).map(&:basename)
+              end
             end
           end
           create_prefix_header