Make sure rewrite rules are written once 'wp_loaded' is fired

[Chouby]

With #30501, it was made sure that an early call to flush_rewrite_rules() would not break rewrite rules.

However, in the context of network activation / deactivation, some plugins don't flush the rewrite rules with flush_rewrite_rules() but with a direct call to delete_option( 'rewrite_rules' );. @jeremyfelt explained the reason for doing this in ​https://jeremyfelt.com/2015/07/17/flushing-rewrite-rules-in-wordpress-multisite-for-fun-and-profit/. For example, Yoast SEO and Polylang are flushing rules at activation / deactivation using this way.

Now, if another plugin uses the rewrite rules before the wp_loaded action is fired (for example by calling url_to_post_id(), the rewrite rules are saved too early and are broken.

Plugins could fix this at activation, but obviously not at deactivation.

Inspired by what has been done to fix #30501, I propose to flush the rewrite rules in the 'wp_loaded' action if there is an attempt to save them before this action has been fired.

[jorbin]

@SergeyBiryukov What do you think of the patch?

[jorbin]

In 38751:

Make sure rewrite rules are not written until wp_loaded has fired

If a plugin attempts to change the rewrite rules to early, other plugins may have their rules inadvertently discarded. Additionally, some function such as url_to_post_id cause a rewrite rule lookup that could cause this accidental flushing. This forces the flushing to only occur once wp_loaded has been fired.

Fixes #37892.
Props Chouby.