By THÉRÈSE MARGOLIS When the international rating agency Moody’s Analytics downgraded Mexico’s state-run oil giant Petróleos Mexicanos credit again earlier this week — this time to out-and-out “junk” with a “negative outlook” — it should have set alarm bells ringing nationwide as to the country’s credibility with investors and lenders. Moody’s new rock-bottom Ba3 rating — based mostly on the