Use new scandir() return value (key is path relative to theme, value is...

Use new scandir() return value (key is path relative to theme, value is absolute path) in WP_Theme->get_page_templates(). Use parent()->get_page_templates() and merge in a parent's page templates, rather than extra logic. see [20312], see #20103.

git-svn-id: https://core.svn.wordpress.org/trunk@20317 1a063a9b-81f0-0310-95a4-ce76da25c4cd