I18N: Add an additional caching layer for `_load_textdomain_just_in_time()`.

Previously, if no translation files exist for a text domain, `_load_textdomain_just_in_time()` went through the entire process each time it was called. This results in an increased call to `get_locale()` and its `locale` filter.
This change splits the logic into `_get_path_to_translation()` and `_get_path_to_translation_from_lang_dir()`. The former, which is used by `_load_textdomain_just_in_time()`, caches the result of the latter. It also removes some non-working code from `WP_Locale_Switcher::load_translations()`.

Props jrf, swissspidy, sharkomatic, ocean90.
Fixes #37997.
Built from https://develop.svn.wordpress.org/trunk@39330


git-svn-id: https://core.svn.wordpress.org/trunk@39270 1a063a9b-81f0-0310-95a4-ce76da25c4cd