Return all the distribution names known to this locator.

def get_distribution_names(self):
        """
        Return all the distribution names known to this locator.
        """
        result = set()
        page = self.get_page(self.base_url)
        if not page:
            raise DistlibException('Unable to get %s' % self.base_url)
        for match in self._distname_re.finditer(page.data):
            result.add(match.group(1))
        return result