Prints out the contents of the import map.

def dump(self):
        """Prints out the contents of the import map."""
        for modpath in sorted(self.map):
            title = 'Imports in %s' % modpath
            print('\n' + title + '\n' + '-'*len(title))
            for name, value in sorted(self.map.get(modpath, {}).items()):
                print('  %s -> %s' % (name, ', '.join(sorted(value))))