Get list of pip installed packages.

def pip_list(self, name=None, prefix=None, abspath=True):
        """Get list of pip installed packages."""
        if (name and prefix) or not (name or prefix):
            raise TypeError("conda pip: exactly one of 'name' ""or 'prefix' "
                            "required.")

        if name:
            prefix = self.get_prefix_envname(name)

        pip_command = os.sep.join([prefix, 'bin', 'python'])
        cmd_list = [pip_command, PIP_LIST_SCRIPT]
        process_worker = ProcessWorker(cmd_list, pip=True, parse=True,
                                       callback=self._pip_list,
                                       extra_kwargs={'prefix': prefix})
        process_worker.sig_finished.connect(self._start)
        self._queue.append(process_worker)
        self._start()

        return process_worker