Store relative-to-wp-content theme roots, as that is what get_theme_roots()...

Store relative-to-wp-content theme roots, as that is what get_theme_roots() should be receiving. see #20103.

Since search_theme_directories() now generates this and leverages this as a cache, convert absolute to relative on cache storage, and relative to absolute on cache retrieval.



git-svn-id: https://core.svn.wordpress.org/trunk@20118 1a063a9b-81f0-0310-95a4-ce76da25c4cd