unload_textdomain() not working for text domains that are loaded just in time

[swissspidy]

#34114 removed the requirement to call load_plugin_textdomain() or load_theme_textdomain() by checking the languages directory for available translation files.

However, as pointed out by @opajaap in the ​support forums, unload_textdomain() does not work anymore because of that. _load_textdomain_just_in_time() would simply load the text domain again and again.

We need some sorts of global array to keep track of unloaded text domains, which _load_textdomain_just_in_time() would ignore. Unloaded text domains would need to be loaded explicitly if one wants to use it again — like before.

Will whip up a patch shortly.

[swissspidy]

Allow manual reloading of text domains

[swissspidy]

37113.2.diff​ extends load_textdomain() to clear unloaded domains from the $l10n_unloaded array so they can be manually loaded again.

[swissspidy]

Reset $l10n_unloaded in tests set up

[swissspidy]

In 37855:

I18N: Enable unloading of text domains that have been loaded just in time.

[37415] removed the requirement to call load_plugin_textdomain() / load_theme_textdomain(). This caused unload_textdomain() to not work properly anymore in these cases.

With this change, unloaded text domains need to be explicitly loaded manually if you want to use them again.

Props swissspidy, ocean90.
Fixes #37113. See #34114.