Being a bit cautious about not turning this into an error since this is
most likely because of case insensitive filesystems.

To keep things simple, especially in getting a `Hash` implementation
correct, I'm leveraging `unicase` for case-insensitive
comparisons which is an existing dependency and I've been using for
years on other projects.

This also opens the door for us to add cross-platform compatibility
hazard warnings about multiple paths that would write to the same
location on a case insensitive file system.  I held off on that because
I assume we would want rust-lang#12235 first.

This does mean we can't test the "no manifest" case anymore because the
one case (no pun intended) I knew of for hitting it is now gone.