If we store the legacy cache files in the supercache directory it will
be much easier to maintain them as they'll be in a directory structure
that reflects the URLs served by the site, just like supercache files.
This patch checks the original blogs_cache directory first but any new
cache files will be created in the supercache directory.

With wp-cache files stored in the supercache directories we can put the
meta files in the same directory since they're separated out by URL.