import-util: tweak url patching helper

import-util: tweak url patching helper

let's share some code between import_url_last_component() and
import_url_change_last_component(), and make sure we never eat up the
hostname component of the URL when parsing out the last component.

Let's also make import_url_change_last_component() more generic so that
we can also use it for append components to paths, instead of replacing
suffixes.