splitdir("/foo/bar") が ("", "foo", "bar") を返す

Unlike just splitting the directories on the separator, empty
directory names ('') can be returned, because these are significant on
some OSes.


Note that this does *not* collapse x/../y sections into y. This is by
design. If /foo on your system is a symlink to /bar/baz, then
/foo/../quux is actually /bar/quux, not /quux as a naive ../-removal
would give you. If you want to do this kind of processing, you
probably want "Cwd"'s "realpath()" function to actually traverse the
filesystem cleaning up paths like this.