A half-mile section of Washington Heights has been transformed into an open-air prescription drug bazaar, with brazen vendors illegally peddling painkillers, erectile dysfunction pills, and an array of other meds, The Post has learned. The sprawling street pharmacy stretches across 10 square blocks, in and around West 179th to West 183rd streets and St. Nicholas Avenue with at least 20 vendors showcasing their wares in loose blister packs, boxes, and bottles strewn across dingy folding tables. “Is it for an adult? You can take up to three a day until they’re all gone,” barked a vendor selling blister packs of…