builtinimport: Set __path__ attribute on packages.

Per https://docs.python.org/3.3/reference/import.html , this is the way to
tell module from package: "Specifically, any module that contains a __path__
attribute is considered a package." And it for sure will be needed to
implement relative imports.

py/builtinimport.c

@@ -228,6 +228,9 @@ mp_obj_t mp_builtin___import__(uint n_args, mp_obj_t *args) {
                     }
                     do_load(module_obj, &path);
                     vstr_cut_tail_bytes(&path, sizeof("/__init__.py") - 1); // cut off /__init__.py
+                    // https://docs.python.org/3.3/reference/import.html
+                    // "Specifically, any module that contains a __path__ attribute is considered a package."
+                    mp_store_attr(module_obj, MP_QSTR___path__, mp_obj_new_str((byte*)vstr_str(&path), vstr_len(&path), false));
                 } else { // MP_IMPORT_STAT_FILE
                     do_load(module_obj, &path);
                     // TODO: We cannot just break here, at the very least, we must execute

py/qstrdefs.h

@@ -13,6 +13,7 @@ Q(__module__)
 Q(__name__)
 Q(__next__)
 Q(__qualname__)
+Q(__path__)
 Q(__repl_print__)
 
 Q(__bool__)