Prints out the contents of the usage map.

def dump(self):
        """Prints out the contents of the usage map."""
        for modpath in sorted(self.map):
            title = 'Used by %s' % modpath
            print('\n' + title + '\n' + '-'*len(title))
            for origin in sorted(self.get_used_origins(modpath)):
                print('  %s' % origin)