Cache the directory of libraries

Can we move this over to #466090: Scan: Cache results ?

I think now that #719896: Add a hook_libraries_info() is in, it doesn't make sense to only cache the directories, but concentrate on caching the whole library information over in #466090: Scan: Cache results.

Please re-open if you feel differently.

Hmmm... #466090: Scan: Cache results is good for the DB cache. But when that is cleared, you're going to end up calling libraries_get_path() possibly a bunch of times on the next page load. So I think we should do static caching in libraries_get_libraries(). Re-opening.

  static $libraries;

  if (!isset($libraries)) {
    $libraries = libraries_get_libraries();
  }

Ehhh.... right. I'm actually pretty much a static-caching noob, but I now that I've found this (...), I don't think there's anything we can do.
(Right?)

Marking fixed, because the new issue statuses scare me. :)