Use the primary `meta_query` clause when parsing `orderby` in `WP_Query`.

When using legacy `meta_key`, `meta_value`, etc. arguments in `WP_Query`,
they're converted into the first clause of a `meta_query`. By using that
clause instead of the original arguments, we make sure that behavior is
consistent between the two available formats.

props boonebgorges.
fixes #16814.

Built from https://develop.svn.wordpress.org/trunk@29855


git-svn-id: http://core.svn.wordpress.org/trunk@29618 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Language packs: Remove translations when deleting a theme or a plugin.

This is for translation files in WP_LANG_DIR which are installed through a language pack.
Change `wp_get_installed_translations()` to only return a translation if the .mo file also exists.

fixes #29860.
Built from https://develop.svn.wordpress.org/trunk@29856


git-svn-id: http://core.svn.wordpress.org/trunk@29619 1a063a9b-81f0-0310-95a4-ce76da25c4cd