Allow install profiles to specify a list of recipes that should be applied

Added this new ability, and a basic test to prove it works with core recipes.

The main missing thing here is path resolution. Core recipe paths are used as-is, but if we add this ability, then we need to resolve recipe directory paths relative to...somewhere. How should we handle that? A few possibilities:

• We could always resolve the paths relative to the web root, so in a drupal/recommended-project layout, the paths would be listed as ../recipes/foo. This is the simplest, but it means profiles using this superpower would be tightly coupled to the project layout.
• We could introspect the Composer configuration for the project to determine where drupal-recipe packages will be installed, and resolve relative to that. This is what Drupal CMS does, and is probably the most robust choice.
• We could do some kind of lightweight "recipe discovery" where we scan certain directories for recipes, at install time only.
• We could add an $install_state['recipes_directory'] variable, defaulting to ../recipes. This could be overridden programmatically by calling install_drupal() directly, or by a custom profile which does a similar thing. This is probably the most lightweight "configurable" option, since $install_state is already a hodgepodge of various state flags used only by the installer.
• We could also do nothing to address this, and leave the patch as-is. This approach would force custom profiles which want to use this superpower to fully resolve the recipe paths on their own and inject them into $install_state['profile_info'].

Or maybe something else?

However if we do that, what happens to core recipes in that case? Should we just special-case the core directory and get everything else from the recipes directory?

Yes. We already explicitly make an allowance for core recipes in the recipe system, so we'd just be doing the same thing here. Either the referenced recipe is a directory relative to the web root, or it's derived from the Composer install location.

Was also wondering here about recipes shipped inside the profile directory.

Is that something we need to support? IMHO, we shouldn't. Recipes are atomic Composer packages, and part of the rationale for that was specifically to avoid the "modules can contain other modules, oh and also the profile can have them too" extension scanning dance inherited from Ye Olden Days.

If a profile wants to include recipes, it needs to reference them as Composer dependencies. It doesn't get to ship them internally. :) Recipes are deliberately Composer-native.

CR written: https://www.drupal.org/node/3612237

Follow-up added: #3612240: Ensure install profiles can apply contributed recipes