imports: Fix Updater importing

updater module cannot be included in 'client' default modules
if updater module also loads 'client' (as python2 fails to import it).

We do want to load client default modules in updater to be able to use
namespacing, e.g. client.download.safe_download().

So don't include updater in default modules: this means the examples
should use e.g.
  from tuf.client import updater
or
  from tuf.client.updater import Updater

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: Jussi Kukkonen

tuf/client/updater.py

@@ -51,10 +51,8 @@
 
 <Example Client>
 
-  # The client first imports the 'updater.py' module, the only module the
-  # client is required to import.  The client will utilize a single class
-  # from this module.
-  from tuf import client
+  # The client first imports the Updater class:
+  from tuf.client.updater import Updater
 
   # The only other module the client interacts with is 'tuf.settings'.  The
   # client accesses this module solely to set the repository directory.
@@ -83,7 +81,7 @@
   # is called with two arguments.  The first argument assigns a name to this
   # particular updater and the second argument the repository mirrors defined
   # above.
-  updater = client.updater.Updater('updater', repository_mirrors)
+  updater = Updater('updater', repository_mirrors)
 
   # The client next calls the refresh() method to ensure it has the latest
   # copies of the metadata files.
@@ -495,7 +493,7 @@ def get_updater(self, repository_name):
 
       else:
         # Create repository mirrors object needed by the
-        # client.updater.Updater().  Each 'repository_name' can have more
+        # Updater().  Each 'repository_name' can have more
         # than one mirror.
         mirrors = {}
 
@@ -510,7 +508,7 @@ def get_updater(self, repository_name):
           # NOTE: State (e.g., keys) should NOT be shared across different
           # updater instances.
           logger.debug('Adding updater for ' + repr(repository_name))
-          updater = client.updater.Updater(repository_name, mirrors)
+          updater = Updater(repository_name, mirrors)
 
         except Exception:
           return None