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Given a ZipProvider, if the underlying ZipImporter prefix is empty,
then resource_listdir('') and resource_listdir('subdir/') fail, while
resource_listdir('/') and resource_listdir('subdir') succeed.

On the other hand, if the underlying ZipImport prefix is not empty,
then resource_listdir('/') fails but resource_listdir('') succeeds.

With this change, the cases listed succeed with or without trailing
slashes.