ast: parse import without package

Before, resolveRefs ignored Imports when Package is nil.
This caused `opa eval --import` option to be ignored without `--package` option.

This PR makes query compiler generate temporary package with name of ""
(empty string) when package is nil but import one or many imports are provided.

Fixes #3228

Signed-off-by: Hiro Osaki <hiroyuki.osaki@gmail.com>