Make IPADiscovery available in PyPI packages

The ipaclient PyPI package does not ship the ipaclient.install
subpackage. The ipaclient.install.ipadiscovery module with IPADiscovery
is now available as ipaclient.discovery, so it can be used by consumers
of PyPI packages.

The module ipaclient.install.ipadiscovery provides a backwards
compatibility shim with deprecation warning.

Fixes: https://pagure.io/freeipa/issue/7861
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>