By Dave Graham and Michael O'Boyle MEXICO CITY (Reuters) – Mexico's government is poised to break a taboo and tax some foods and medicine under an tax overhaul proposal but will take measures that shield the country's poorest, ruling party lawmakers said on Tuesday. The bill will also very likely propose raising the top income tax rate for the wealthiest to as much as 40 percent from 30 percent, several officials from President Enrique Pena Nieto's Institutional Revolutionary Party (PRI) said on condition of anonymity. …