Display links to help topics provided by uninstalled modules

Problem/Motivation

In reviewing 📌 Merge Help Topics classes into Help with BC layer Fixed , we noticed the following comment and @todo in the HelpTopicPluginManager::findDefinitions() method:

// If the related help topic does not exist it might be for a module
        // that is not installed. Remove it.
        // @todo Discuss this more as this could cause silent errors but it
        //   offers useful functionality to relate to a help topic provided by
        //   extensions that are yet to be installed.

What this is saying is that it would be helpful if help topics for uninstalled modules were listed and accessible as related topics and top-level topics. (We already have a solution in place for topics linked within a help topic, through the use of help_topic_link().)

As as site admin, you might be looking for help for how to configure your site, but you can't find the help you need because the module you need isn't installed and the help topic describing the solution to your problem is not showing up anywhere (in help search, as a related topic, or as a top-level topic on admin/help). This is partially solved through help_topic_link(), if you are reading a topic that is related to your problem, or if you use search, you will come across the text -- not linked. It may or may not be obvious which module you need to install to view that help topic in the UI.

As mentioned, we've worked around this, through the custom Twig function help_topic_link(), which will display the link text but not the link itself if, if the linked topic belongs to a module that is not installed.

This issue is specifically about the merits of listing topics from uninstalled modules as top-level (at admin/help) or as related topics (in a topic itself).

Steps to reproduce

We'll use a topic from Actions UI module to reproduce, as it is uninstalled out-of-the-box.

1. Install Drupal
2. Install Help Topics, or if Help Topics has been merged with Help, ensure the core Help module is installed.
3. Go to admin/help. Under Topics, notice that there is not a topic "Configuring actions" listed there. (This is what we'd like to address in this issue.)
4. Using the Help search field, search for "Configuring actions".
5. Click on the search result, "Adding a bulk operations form to a view"
6. In the Goal of this topic, notice a reference to the "Configuring actions" topic, along with a note about installing Actions module to view this topic. (This is all good.)
7. Under Related topics, notice that there is not a topic "Configuring actions" listed there. (This is also what we'd like to address in this issue.)
8. Return to the Extend page and install Actions UI module. Repeat the steps above (3-7) and notice that a) Configuration actions appears as a top-level topic at admin/help; b) (After running cron or otherwise updating the search index) Searching for "Configuring actions" also returns a result for the corresponding topic; c) Revisiting the bulk operations topic, you will see that in the Goal, the text "Configuration actions" is linked, and the topic appears under Related topics. Also the other topics provided by Actions UI are listed (i.e. "Creating an advanced action").

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

API changes

Data model changes

Release notes snippet