do not rely on pkg_resources (#46476)

README

@@ -20,6 +20,13 @@ the django-mellon adapter:
 And appropriate django-mellon parameters (MELLON_PUBLIC_KEYS,
 MELLON_PRIVATE_KEY, MELLON_IDENTITY_PROVIDERS).
 
+You also have to register the plugin :
+
+  import authentic2_auth_fedict
+
+  INSTALLED_APPS, TENANT_APPS, AUTHENTICATION_BACKENDS, AUTH_FRONTENDS = authentic2_auth_fedict.register(
+      INSTALLED_APPS, TENANT_APPS, AUTHENTICATION_BACKENDS, AUTH_FRONTENDS
+  )
 
 Code Style
 ----------

src/authentic2_auth_fedict/__init__.py

@@ -35,6 +35,9 @@ class AppConfig(django.apps.AppConfig):
                 user = kwargs.get('user')
                 user.backend = 'authentic2_auth_fedict.backends.FedictBackend'
 
+    def get_a2_plugin(self):
+        return Plugin()
+
 
 default_app_config = 'authentic2_auth_fedict.AppConfig'
 
@@ -45,18 +48,6 @@ class Plugin:
 
         return urls.urlpatterns
 
-    def get_apps(self):
-        return ['mellon', __name__]
-
-    def get_authentication_backends(self):
-        return ['authentic2_auth_fedict.backends.FedictBackend']
-
-    def get_auth_frontends(self):
-        return ['authentic2_auth_fedict.authenticators.FedictAuthenticator']
-
-    def get_authenticators(self):
-        return ['authentic2_auth_fedict.authenticators.FedictAuthenticator']
-
     def redirect_logout_list(self, request, next_url=None):
         from mellon.views import logout
 
@@ -147,3 +138,19 @@ class Plugin:
                 'field_class': fields.CountryField,
             },
         ]
+
+
+def register(installed_apps, tenant_apps, authentication_backends, auth_frontends):
+    apps = []
+    for app in ('mellon', 'authentic2_auth_fedict'):
+        if app not in installed_apps:
+            apps.append(app)
+    if apps:
+        installed_apps += tuple(apps)
+        tenant_apps += tuple(apps)
+    if 'authentic2_auth_fedict.backends.FedictBackend' not in authentication_backends:
+        authentication_backends += ('authentic2_auth_fedict.backends.FedictBackend',)
+    if 'authentic2_auth_fedict.authenticators.FedictAuthenticator' not in auth_frontends:
+        auth_frontends += ('authentic2_auth_fedict.authenticators.FedictAuthenticator',)
+
+    return installed_apps, tenant_apps, authentication_backends, auth_frontends