Don't purge mods-available dir when separate enable dir is used

Purging /etc/apache2/mods-available on Debian when there are separate available/enabled directories is poor behaviour, since this isn't active configuration.  When purged, you lose track of which modules are available but that this module doesn't handle.

This patch disables the purge behaviour on the module config directory, but only when there is also a separate module enabled directory.