Autoloader: Detect filtering of active_plugins

[ObliviousHarmony]

Changes proposed in this Pull Request:

In woocommerce/woocommerce#27608 it became apparent that plugins can filter active_plugins in a way that prevents some plugins from being detected by the autoloader. This PR addressed the issue by caching the list in should_autoloader_reset() as an additional means to evaluating whether or not manifests should be processed again.

Jetpack product discussion

N/A

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Add this autoloader-testing.php file to mu-plugin. It sets up autoloader globals in a way that pretends the autoloader has been successfully loaded. Without this PR this will result in the entire site becoming inaccessible due to class loading errors, but with this PR, it will work as-expected.

<?php
/**
 * Plugin Name: Autoloader Testing
 * Description: This plugin helps.
 * Version: 1.0
 */

 defined( 'ABSPATH' ) || exit;

// Set a version for the autoloader so that Jetpack won't attempt to
// reset the autoloader.
global $jetpack_autoloader_latest_version;
$jetpack_autoloader_latest_version = '2.2.0.0';
global $jetpack_autoloader_cached_plugin_paths;
$jetpack_autoloader_cached_plugin_paths = array();

Proposed changelog entry for your changes:

• Autoloader: Added handling for filtered active_plugins options that would otherwise have left classes out.

[jetpackbot]

Scheduled Jetpack release: October 6, 2020.
Scheduled code freeze: September 29, 2020

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-17174

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Generated by 🚫 dangerJS against 948fa32

[kbrown9]

This looks great to me! Thanks for the test plugin.

It looks like there's a problem with the E2E tests which has nothing to do with this PR, and we're working on a PR to fix them. We'll rebase/retest/merge this PR as soon as that's fixed.