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?