Ensure components for the demo are available after installation

We generally install commerce_demo as a part of the whole parent distro, the Composer demo-project:
https://github.com/drupalcommerce/demo-project

Other installation methods will be perfected over time. We'll probably need separate issues for commerce_base and minimal. I am surprised that commerce_base doesn't work, since it's mostly a clone of standard with the Commerce modules added to the install list.

Exploring this a bit.

The profile in the Composer template provides block placement and sample pages. This is the glue which allows proper placement of blocks for the demo.

It discussion with bojanz, it would make sense to just place Belgrade specific configuration into the config/optional directory, removing the need to use the template. However, that does not help with Bartik or other future themes the demo could support "out of the box" (or themes built for the demo.)

An idea would be a "theme installer service" to dynamically place blocks in proper regions for more custom themes. Instead of maintaining hundreds of optional config. This would invoke on commerce_demo_modules_installed when the module itself is installed, or whenever a theme is installed.

EDIT: This is blocked on #2988636: Automatically swap out the default cart block plugin definition's class.. The Belgrade cart block config targets that plugin. Instead it should target the one provided by Commerce itself.

This is an initial patch with some basic testing. Whenever the module is installed, blocks are set up for the active theme. Whenever a theme is installed, its blocks are set up and placed.

I need to test with a basic installation and changing themes on an active site.

Handles changing a theme when the site is installed. Note

      // When changing themes, the `block_themes_installed` hook runs first,
      // and invokes block_theme_initialize. This caused blocks from the
      // previous theme to carry over. We ensure, here, their placement in the
      // event the block was moved to the "default" region.

I tested Method 3 as mentioned. It works as documented, but no blocks placed.

:) because the active theme is "seven", I need to check $default_theme = \Drupal::config('system.theme')->get('default');

Fixes scenario 3. Working on 2 (minimal install)

While testing, I wasn't able to reproduce the issues on the minimal install.

But we should also document:

Installing the module after a clean Drupal core installation can be a demanding operation for your laptop, desktop, webserver. It is recommended to expand PHP's minimum execution time to at least 300 seconds.

Updated patch after discussing w/ bojanz.

Also retitling. The main focus is ensuring parts of the demo exist regardless if it is installed through our demo template or not.

1. +++ b/commerce_demo.module
   @@ -59,6 +59,20 @@ function commerce_demo_modules_installed($modules) {
   +    $theme_installer = \Drupal::getContainer()->get('commerce_demo.theme_handler');
   ...
   +    $theme_installer->placeBlocks($default_theme);
   ...
   +  $theme_installer = \Drupal::getContainer()->get('commerce_demo.theme_handler');
   ...
   +    $theme_installer->placeBlocks($theme);
   

   theme_handler

2. +++ b/src/ThemeHandler.php
   @@ -0,0 +1,162 @@
   +    // The commerce_base profile already provides a shopping cart block.
   +    if ($this->blockStorage->load('shopping_cart')) {
   +      unset($data['shopping_cart']);
   +    }
   +    else {
   +      $data['shopping_cart']['region'] = 'header';
   +      $data['shopping_cart']['weight'] = NULL;
   +    }
   

   We should account for this in the main block definitions when placing blocks.
   It can be removed, we account for existing blocks.

New patch. Also ensures block is enabled.

Opened followup: #2990498: Can't be installed together with Belgrade on a regular install profile.