This already raised if the order was [foo, foo/bar]. But it didn't
prevent adding [foo/bar, foo].
The only sub-classes of FileRegistry are FileCopier and Jarrer.
FileCopier.copy threw in the previously unhandled case: the order of
creation is the same as the order of addition, so that foo is created
after foo/bar.
A zip file index can contain both foo and foo/bar. I don't think we
should rely on this property in our use of Jarrer, but if we already do,
I guess we need to move these guards into FileCopier. Let's hope that's
not the case!
(For the record: On my Mac OS X system, unzipping such a zip file
prompts the user for what to do, depending on the order of the entries
in the zip index.)
