Rewrite search_theme_directories() -- better performance and allow for caching.

 * Update it to use PHP5 scandir().
 * Don't scan individual theme dirs -- we only need to check for style.css. (Solid performance gains.)
 * Improve and simplify branching.
 * Introduce wp_cache_themes_persistently filter to enable persistent caching of the return value, based on the theme_roots transient.

see #20103.


git-svn-id: https://core.svn.wordpress.org/trunk@20020 1a063a9b-81f0-0310-95a4-ce76da25c4cd