Add a file to the manifest.

        :param item: The pathname to add. This can be relative to the base.

def add(self, item):
        """
        Add a file to the manifest.

        :param item: The pathname to add. This can be relative to the base.
        """
        if not item.startswith(self.prefix):
            item = os.path.join(self.base, item)
        self.files.add(os.path.normpath(item))