plugable: replace API.import_plugins with new API.add_package

Replace API.import_plugins with a new method API.add_package which allows
loading plugin packages into an API object from a package object.

This makes loading of plugin packages loading consistent with loading of
plugin modules and classes.

Rename API.modules to API.packages and use package objects where
implemented to reflect the change.

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

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

ipaserver/advise/base.py

@@ -127,7 +127,11 @@ class Advice(Plugin):
 
 class AdviseAPI(API):
     bases = (Advice,)
-    modules = ('ipaserver.advise.plugins.*',)
+
+    @property
+    def packages(self):
+        import ipaserver.advise.plugins
+        return (ipaserver.advise.plugins,)
 
 advise_api = AdviseAPI()