RecipeActivator needs to handle unpacked recipes that aren't Composer-managed

Problem/Motivation

Core now has a recipe unpacking plugin which moves a recipe's dependencies into the top-level composer.json at require time, and then removes the recipe from Composer's jurisdiction. The recipe is still physically there, but Composer no longer knows about it. That's a problem, because RecipeActivator relies on Composer to tell it where a given recipe is installed.

Proposed resolution

RecipeActivator should first ask Composer about where a recipe is installed, and if Composer doesn't know, it should try to compute the path based on what can be read from composer.json.