public void setUrl(String url) {
        ensureNotFrozen();
        this.url=url;
    }
Sets the url of this source. If a source has an url (i.e this returns non-null), the content of
the url is <i>not</i> fetched - fetching is left to the frontend by exposing this url in the result.
container-search/src/main/java/com/yahoo/search/pagetemplates/model/Source.java