Try yet again try to fix bug #9158.

Try yet again try to fix bug #9158.

The problem with the previous attempt was that, every time through
updateBuffer, we looked up the file location using kpsewhich, which
took too long on Windows. The new solution is to cache that info, and
to look it up only when we need it.

Previously, this info would have been re-read whenever we parsed the
bibfiles. So we re-read it now whenever the bibinfo cache is invalid,
which is less often, but should be good enough. We can add more such
re-reads if need be.