Clear available pkgs cache when available packages are likely to have changed

yumpkg.py stores the information on available packages in ``__context__`` to avoid re-performing this data collection. However, if the configured repositories have changed (or if packages are installed/removed, since packages can provide repos), then this cache data may be outdated.

This pull req clears the variable from ``__context__`` in these cases, forcing ``check_db()`` to re-gather this information on its next run.