[2.1] Ensure services that are delayed and have requirements are scheduled correctly

[westonruter]

Summary

Cherry-picks #6548 onto the 2.1 branch, with the exception of 1db8a17 which is not relevant to the 2.1 branch which does not include the changes from #6518.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

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[pierlon]

QA Passed

To test this change, I first had to modify the plugin to enable service filtering:

diff --git a/src/AmpWpPlugin.php b/src/AmpWpPlugin.php
--- a/src/AmpWpPlugin.php	(revision 87e6a798bcf5c92bd44872453def3c21af33f823)
+++ b/src/AmpWpPlugin.php	(date 1630414970084)
@@ -47,7 +47,7 @@
 	 */
 
 	// Whether to enable filtering by default or not.
-	const ENABLE_FILTERS_DEFAULT = false;
+	const ENABLE_FILTERS_DEFAULT = true;
 
 	/**
 	 * Prefix to use for all actions and filters.

Then I installed the following plugin:

<?php

/**
 * Plugin Name: Services Test (QA 6553)
 */

namespace ServicesTest;

use AmpProject\AmpWP\Infrastructure\Delayed;
use AmpProject\AmpWP\Infrastructure\HasRequirements;
use AmpProject\AmpWP\Infrastructure\Registerable;
use AmpProject\AmpWP\Infrastructure\Service;

class Service_A implements Service, Delayed {

	public static function get_registration_action() {
		return 'init';
	}
}

class Service_B implements Service, Delayed {

	public static function get_registration_action() {
		return 'wp_loaded';
	}
}

class Service_C implements Service, HasRequirements, Delayed, Registerable {

	protected $registered_action;

	public static function get_requirements() {
		return [ 'service_a', 'service_b' ];
	}

	public static function get_registration_action() {
		return 'admin_init';
	}

	public function register() {
		$this->registered_action = current_action();
		add_action( 'admin_notices', [ $this, 'render_notice' ] );
	}

	public function render_notice() {
		$notice_type = 'admin_init' === $this->registered_action ? 'notice-info' : 'notice-error';
		?>
		<div class="amp-plugin-notice notice <?php echo $notice_type ?>" id="service-c-notice">
			<p>Service C was registered at <code><?php esc_html_e( $this->registered_action ) ?></code>.</p>
		</div>
		<?php
	}
}

add_filter(
	'amp_services',
	static function ($services) {
		return array_merge(
			$services,
			[
				'service_a' => Service_A::class,
				'service_b' => Service_B::class,
				'service_c' => Service_C::class,
			]
		);
	}
);

After activating the plugin, here is the before and after showing at what action Service_C was scheduled:

Before	After