plugable: simplify API plugin initialization code

Use a flat dictionary to track plugins in API rather than nested
dictionaries.

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

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

ipalib/plugable.py

@@ -574,16 +574,18 @@ class API(ReadOnly):
             raise TypeError('plugin must be a class; got %r' % klass)
 
         # Find the base class or raise SubclassError:
-        found = False
         for base in self.bases:
-            if not issubclass(klass, base):
-                continue
-
-            sub_d = self.__plugins.setdefault(base, {})
-            found = True
+            if issubclass(klass, self.bases):
+                break
+        else:
+            raise errors.PluginSubclassError(
+                plugin=klass,
+                bases=self.bases,
+            )
 
         # Check override:
-            if klass.__name__ in sub_d:
+        prev = self.__plugins.get(klass.__name__)
+        if prev:
             if not override:
                 # Must use override=True to override:
                 raise errors.PluginOverrideError(
@@ -601,13 +603,7 @@ class API(ReadOnly):
                 )
 
         # The plugin is okay, add to sub_d:
-            sub_d[klass.__name__] = klass
-
-        if not found:
-            raise errors.PluginSubclassError(
-                plugin=klass,
-                bases=self.bases,
-            )
+        self.__plugins[klass.__name__] = klass
 
     def finalize(self):
         """
@@ -625,10 +621,11 @@ class API(ReadOnly):
 
         for base in self.bases:
             name = base.__name__
-            sub_d = self.__plugins.get(base, {})
-
             members = []
-            for klass in sub_d.values():
+
+            for klass in self.__plugins.values():
+                if not issubclass(klass, base):
+                    continue
                 try:
                     instance = plugins[klass]
                 except KeyError: