ipalib: move server-side plugins to ipaserver

Move the remaining plugin code from ipalib.plugins to ipaserver.plugins.

Remove the now unused ipalib.plugins package.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipatests/test_xmlrpc/test_selinuxusermap_plugin.py

@@ -17,7 +17,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """
-Test the `ipalib/plugins/selinuxusermap.py` module.
+Test the `ipaserver/plugins/selinuxusermap.py` module.
 """
 
 from ipalib import api, errors