None of them work. Maybe I'm doing something wrong.
I have my project file (say, "main.tex") in a folder (say, "Main") and projects in a subfolder (say "Products").
I've written some Lua macros in a file (say, "foo.lua") and I need all the products to be able to call those Lua functions, so I tried loading them in the project file ("foo.lua" and "main.tex" are in the same folder, whereas products are in the subfolder). \usepath works for finding other files, such as images, so I don't know what is going on. I'm attaching a test project.