Recipe validation should always treat required modules as installed

Problem/Motivation

Right now, the recipe system validates everything in the config.actions section of recipe.yml, trying to confirm that every module mentioned in that section (i.e., in the prefix of the config entities' names) is either installed already, or will be installed by a recipe in the stack.

That's a bit too strict -- required modules should not need to be listed, because they are always guaranteed to be installed. Fixing this will allow Recipe objects to be instantiated during the early installer, which is important for Drupal CMS.

Proposed resolution

In \Drupal\Core\Recipe\Recipe::validateConfigActions(), treat required modules as installed, even if they're not. Because we know they will be.