XPI import fails to descend into jar files

A bug in XPI manifest handling that stops the importer from descending into jar files.

When encountering a DTD or properties file, the importer asks the manifest: "do you have a locale entry whose path is a prefix of this file's path?" This is correct. But when encountering a jar file, the right question to ask the manifest is "do you have any locale entries that have this path as a prefix?"

This requires an API extension to the manifest class. It can be tested by adding a manifest to one of the existing XPI import tests that use jar files.