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/init/plugins/__init__.py

@@ -15,6 +15,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import logging
+import sys
 
 from mediagoblin import mg_globals
 from mediagoblin.tools import pluginapi
@@ -36,24 +37,21 @@ def setup_plugins():
         _log.info("No plugins to load")
         return
 
-    pcache = pluginapi.PluginCache()
+    pman = pluginapi.PluginManager()
 
     # Go through and import all the modules that are subsections of
-    # the [plugins] section.
+    # the [plugins] section and read in the hooks.
     for plugin_module, config in plugin_section.items():
         _log.info("Importing plugin module: %s" % plugin_module)
+        pman.register_plugin(plugin_module)
         # If this throws errors, that's ok--it'll halt mediagoblin
         # startup.
         __import__(plugin_module)
+        plugin = sys.modules[plugin_module]
+        if hasattr(plugin, 'hooks'):
+            pman.register_hooks(plugin.hooks)
 
-    # Note: One side-effect of importing things is that anything that
-    # subclassed pluginapi.Plugin is registered.
-
-    # Go through all the plugin classes, instantiate them, and call
-    # setup_plugin so they can figure things out.
-    for plugin_class in pcache.plugin_classes:
-        name = plugin_class.__module__ + "." + plugin_class.__name__
-        _log.info("Loading plugin: %s" % name)
-        plugin_obj = plugin_class()
-        plugin_obj.setup_plugin()
-        pcache.register_plugin_object(plugin_obj)
+    # Execute anything registered to the setup hook.
+    setup_list = pman.get_hook_callables('setup')
+    for fun in setup_list:
+        fun()