Rework plugin infrastructure to nix side-effects

This reworks the plugin infrastructure so as to remove module-loading
side-effects which were making things a pain in the ass to test.

With the new system, there's no auto-registering meta class. Instead
plugins do whatever they want and then specify a hooks dict that maps
hook names to callables for the things they're tying into. The most
common one (and the only one we've implemented so far) is "setup".

This also simplifies the sampleplugin a little by moving the code
to __init__.py.

mediagoblin/plugins/sampleplugin/__init__.py

@@ -15,6 +15,28 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
-# This imports the module that has the Plugin subclass in it which
-# causes that module to get imported and that class to get registered.
-import mediagoblin.plugins.sampleplugin.main
+import logging
+
+from mediagoblin.tools.pluginapi import get_config
+
+
+_log = logging.getLogger(__name__)
+
+
+_setup_plugin_called = 0
+
+def setup_plugin():
+    global _setup_plugin_called
+
+    _log.info('Sample plugin set up!')
+    config = get_config('mediagoblin.plugins.sampleplugin')
+    if config:
+        _log.info('%r' % config)
+    else:
+        _log.info('There is no configuration set.')
+    _setup_plugin_called += 1
+
+
+hooks = {
+    'setup': setup_plugin
+    }

mediagoblin/plugins/sampleplugin/main.py

@@ -1,42 +0,0 @@
-# GNU MediaGoblin -- federated, autonomous media hosting
-# Copyright (C) 2011, 2012 MediaGoblin contributors.  See AUTHORS.
-#
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU Affero General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU Affero General Public License for more details.
-#
-# You should have received a copy of the GNU Affero General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-import logging
-from mediagoblin.tools.pluginapi import Plugin, get_config
-
-
-_log = logging.getLogger(__name__)
-
-
-class SamplePlugin(Plugin):
-    """
-    This is a sample plugin class. It automatically registers itself
-    with mediagoblin when this module is imported.
-
-    The setup_plugin method prints configuration for this plugin if
-    it exists.
-    """
-    def __init__(self):
-        self._setup_plugin_called = 0
-
-    def setup_plugin(self):
-        _log.info('Sample plugin set up!')
-        config = get_config('mediagoblin.plugins.sampleplugin')
-        if config:
-            _log.info('%r' % config)
-        else:
-            _log.info('There is no configuration set.')
-        self._setup_plugin_called += 1