Restructure our python module. A lot.

src/pretix/base/plugins.py

@@ -0,0 +1,23 @@
+try:  # NOQA
+    from enum import Enum
+except ImportError:  # NOQA
+    from flufl.enum import Enum  # remove this dependency when support for python <=3.3 is dropped
+
+from django.apps import apps
+
+
+class PluginType(Enum):
+    RESTRICTION = 1
+
+
+def get_all_plugins() -> "List[class]":
+    """
+    Returns the PretixPluginMeta classes of all plugins found in the installed Django apps.
+    """
+    plugins = []
+    for app in apps.get_app_configs():
+        if hasattr(app, 'PretixPluginMeta'):
+            meta = app.PretixPluginMeta
+            meta.module = app.name
+            plugins.append(meta)
+    return plugins