Fix make clean.

Fixes two issues with make clean:

• Performance: the cook.sh, and the unfetch.sh and fetch.sh scripts, currently iterate over the recipe names as opposed to the complete recipe paths, forcing them to use find_recipe each time. However, find_recipe looks up the path also by scanning all recipes, resulting in very inefficient O(n^2) scaling, with already becomes obvious considering there are >1500 recipes currently. This MR linearizes the search, improving make clean time from 2 min 45 s (on an NVME SSD filesystem), to < 20 seconds.
• Duplicate recipe leaf names: make clean on master, fails because both recipes/wip/tools/nginx and recipes/wip/net/nginx exist.