Internally cache themes inside wp_get_themes() by theme_root as well as...

Internally cache themes inside wp_get_themes() by theme_root as well as stylesheet, to avoid conflicts with future calls to wp_get_themes(). Always return only the last stylesheet found, as before. see #20103.

git-svn-id: https://core.svn.wordpress.org/trunk@20375 1a063a9b-81f0-0310-95a4-ce76da25c4cd