Find the full paths of files in modules
Note
If called from outside a module, the current working directory is used.
This function is similar to system.file for packages. Its semantics
differ in the presence of non-existent files: box::file always returns
the requested paths, even for non-existent files; whereas system.file
returns empty strings for non-existent files, or fails (if requested via the
argument mustWork = TRUE).
