Function find_module

Given the path to a source file containing a `mod` item, and the name of the module, attempts to find the source file which contains that module. It does this by trying first to find a file `$MODNAME.rs` in the same directory as `source_path`, and failing that will look for `$MODNAME/mod.rs`. If either of those exises, the path to the file is returned, but no attempt is made to determine if it's

(source_path: &Path, module_name: &str)